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1. Introduction
Morey (1) defined glass as ‘‘an inorganic substance in a condition that is continuous with the liquid state, but which, as the result of a reversible change in viscosity during cooling, has attained so high a degree of viscosity as to be, for all
practical purposes, rigid.’’ The American Society for Testing and Materials
ASTM (2) defines glass as ‘‘an inorganic product of fusion that has been cooled
to a rigid condition without crystallizing.’’ However, these definitions do not
explicitly address the character of a noncrystalline structure and the glasstransformation behavior, two characteristics that separate glasses from other
solids. In addition, glasses may be made by processes that do not necessarily produce liquids and so Shelby’s definition seems most appropriate: Glass is a solid
that possesses no long-range atomic order and, upon heating, gradually softens to
the molten state (3).
In principle, any melt forms a glass if cooled so rapidly that insufficient time
is provided to allow reorganization of the structure into crystalline (periodic)
Su
pe
liq rco
ui ole
d d
Li
qu
id
Transformation
range
L
Volume
A
Fa
C
Sl
ow
-c
st-
le
oo
co
d
ole
la
dg
gla
ss
ss
B
Tm
A
Crys
tal
Tg
T ′g
Melting
point
Temperature
Fig. 1. Volume–temperature relationships for glasses, liquids, supercooled liquids, and
crystals.
Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
566
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arrangements. When a liquid is cooled, its volume generally decreases until the
melting point, Tm, is reached and then the volume changes abruptly as the liquid
transforms into a crystalline solid (line A in Fig. 1. If a glass-forming liquid is
cooled below Tm (line B) without the occurrence of crystallization, it is considered
to be a supercooled liquid until the glass-transition temperature, Tg, is reached.
(The Tg is the point below which the viscoelastic melt loses its liquid properties
and the material behaves as a solid.) At temperatures below Tg, the material
becomes a solid glass. Faster cooling leads to a greater Tg and a less dense
glass (line C).
2. Fundamentals
2.1. Kinetic Theory of Glass Formation. Since most glasses are
formed by quenching a melt from some temperature above the material’s crystallization temperature down through the Tg and into the solid state, it is appropriate to discuss glass formation in terms of crystallization kinetics. That is, glass
formation will occur if crystallization can be avoided when a melt is cooled, from
T > Tm to T < Tg.
Classical nucleation and crystallization theory can be used to understand
the conditions that promote glass formation (4). For a crystal nucleus to form,
the atoms in a melt must reorganize to form an ordered crystal structure. In a
supercooled melt at a temperature below the Tm of the crystal, crystalline solids
are thermodynamically preferred over the disorganized melt. However, the surface energy, g, required to create crystal nuclei is a thermodynamic barrier to the
reorganization of the melt atoms into ordered structures, and the increasing melt
viscosity, , with decreasing temperature is a kinetic barrier. The dependence of
the nucleation rate, I, on temperature, T, can be represented by
I¼
A
exp
B 3
Gv T
ð1Þ
where A and B are constants and DGv represents the free energy difference
between the liquid and crystal. Crystals will form from stable nuclei in melts
below Tm at a rate (U) given by
U¼
CT
Gv
1 exp
kT
ð2Þ
where C is a constant and k is Boltzmann’s constant.
A schematic representation of the temperature dependences of nucleation
and crystallization rates described by equations (1) and (2) is shown in
Figure 2. For glass formation to occur, a melt must be quenched from a temperature above Tm to below Tg while avoiding significant (detectable) crystallization.
The increase in melt viscosity as the melt is cooled below Tm counters the free
energy gained by crystallization so that nucleation and crystal growth are no
longer kinetically significant at temperatures below the Tg. As a result, materials
like SiO2 that possess high viscosities (106 Pas) at Tm are easily quenched to
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567
Fig. 2. Schematic representation of the classical rates for nucleation I, and crystallization, U, for a supercooled melt between the melting point, Tm and the glass-transition
temperature, Tg.
form glasses, whereas materials like water that possess low viscosities
(10-3 Pas) at Tm form glasses only under extreme conditions.
Time–temperature transformation (TTT) diagrams (Fig. 3) are another
way to represent the relationships between quenching rates and glass formation.
Crystal growth kinetics can be used to predict the times and temperatures necessary to convert some fraction of a supercooled melt to crystallized material.
Curve A in Figure 3 represents a material with a low viscosity at Tm and
curve B represents a material with a high viscosity at Tm. The latter material
will require longer times at a fixed temperature to acquire the same level of crystallization. To avoid the levels of crystallization represented by the curves, the
two melts must be quenched at rates faster than those represented by the dashed
lines between Tm and the respective ‘‘nose temperatures’’ of the crystallization
curves. The high viscosity melt represented by line B will require a lower critical
Fig. 3. Time–temperature transformation curves representing supercooled melts with
low (curve A) and high viscosities (curve B) at Tm. The dashed lines represent the critical
cooling rates to avoid crystallization.
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quenching rate to avoid crystallization, and so will be a classified as a better
glass-forming material.
To summarize, glass formation will occur when a melt is quenched to temperatures below the Tg at a rapid enough rate to avoid detectible crystallization.
Melts with high viscosities at the crystal melting temperature (or melts that
exhibit rapid viscosity increases when cooled below Tm) are most easily quenched
Table 1. Main Inorganic Glass Systemsa
One element
B, C, P, As, S, Se
group 16 (VIA)
chalcogenidesb
oxides
glass formers
intermediate
formers
sulfides
tellurides
three-element
chalcogenide
systems
group 17 (VIIA)
halides
fluorides
glass formers
intermediate
formers
GeTe2, Si2Te3,
GaTe, GeTe,
SnTe, As2Te3
Ge–As–S, Ge–
As–Se, Ge–
As–Te, Ge–
Sb–Se, As–
Sb–Se, As–
S–Se, As–S–
Te, As–Se–
Te, S–Se–Te
glass formers
intermediate
formers
chlorides
bromides
complex glasses and
nonsilicate glasses
a
glass formers
intermediate
formers
P2O5, As2O5, SiO2, GeO2,
B2O3
MoO3, WO3, TiO2, Fe2O3,
Al2O3, Ga2O3, Bi2O3,
BeO, PbO, Nb2O5,
Ta2O5
As2S5, SiS2, GeS2, B2S3,
Al2S3, Ge2S3, As2S3,
Sb2S3
P2S5, Ga2S3, In2S3
BeF2
ZrF4, HfF4, ThF4, UF4,
ScF3, YF3, CrF3, FeF3,
AlF3, GaF3, InF3, ZnF2,
CdF2, PbF2
ZnCl2
ThCl4, BiCl3, CdCl2,
SnCl2, PbCl2, CuCl,
AgCl, TlCl
ZnBr2
PbBr2, CuBr, AgBr
glass formers
intermediate
formers
iodides
glass formers
ZnI2
intermediate
CdI 2, PbI2, CuI, AgI
formers
oxyhalides, chalcohalides (Se–Te–X; XCl, Br, I), oxyfluorophosphates, oxynitrides (ie, SiAlON glasses), carbonates, nitrates,
nitrites, sulfates, selenates, alkali dichromates
adapted from Ref. 5.
Chalcogen(ide) refers to any of the elements oxygen, sulfur, selenium, and tellurium. However,
oxide-based glasses have far more commercial and technological importance today than any other
chalcogenide-based glass. See also the section on Chalcogenide Glasses.
b
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569
to form glasses. Certain materials possess lower critical cooling rates and so more
readily form glasses. These glass-forming materials are described in subsequent
sections.
2.2. Structural Descriptions of Glass-Forming Systems. Inorganic
glasses are readily formed from a wide variety of materials, principally oxides,
chalcogenides, halides, salts, and combinations of each. Table 1 summarizes
the more common inorganic glass systems.
There have been many attempts to relate the glass-forming tendency of a
material to its molecular level structure. For example, Goldschmidt (6) observed
that oxide glasses with the general formula RnOm form most easily when the
ionic radius ratio of the metal cation and the oxygen ion lies in the range between
0.2 and 0.4. Zachariasen (7) noted that those crystalline oxides that form open,
continuous networks tended to form glasses and those glass-forming networks
were associated with ions with particular coordination numbers (CN). Zachariasen
proposed that the structure of glass was similar to that of a crystal, but with a
larger lattice energy resulting from the disordered arrangements of polyhedral
units, to possess a random network lacking long-range periodicity, as shown
schematically in Figure 4 (7). Zachariasen listed four conditions for a structure
to favor glass formation: (1) an oxygen or anion must not be linked to more than
two cations; (2) the number of oxygens or anions coordinated to the cations must
be small, typically three or four; and (3) the cation–anion polyhedra must share
corners rather than edges or faces; (4) at least three corners must be shared.
These conditions lead to the open structures that can accommodate a distribution
of interpolyhedral bond angles that are associated with the loss of long-range
structural order when a crystal form a glass. Subsequent diffraction studies by
Warren (8) and others (ie, Ref. 9) confirmed Zachariasen’s prediction that glasses
Fig. 4. Schematic two-dimensional (2D) representation of the silica random network
built by SiO4 tetrahedra: (a) crystalline structure (or long-range order), (b) random
network (7).
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and crystals possess similar short-range polyhedral structures but different longrange polyhedral arrangements.
Oxides that do not possess the open network structures of the glass-forming
oxides are sometimes classified as network modifiers or intermediate oxides,
depending on their structural roles (Table 2). Oxides with large coordination
numbers and relatively weak bonds are called network modifiers and they
alter the glass-forming network by replacing stronger bridging oxygen (BO)
bonds between glass-forming polyhedra with weaker, nonbridging oxygen
Table 2. Bond Strengths and Coordination Number of Oxides of Technological
Significancea
Oxide
Glass former
B2O3
SiO2
GeO2
P2O5
V2O5
As2O5
Sb2O5
Intermediates
TiO2
ZnO
PbO
Al2O3
ThO2
BeO
ZrO2
CdO
Modifiers
Sc2O3
La2O3
Y2O3
SnO2
ThO2
PbO2
MgO
Li2O
PbO
ZnO
BaO
CaO
SrO
CdO
Na2O
K2O
Rb2O
Cs2O
a
Ref. 10
Dissociation
energy,
kJ/mol
Coordination
number
Single-bond
strength,
kJ/mol
CAS
Registry
number
1489
1774
1803
1849
1879
1460
1418
3; 4
4
4
4
4
4
4
496; 372
443
450
462–370
469–376
365–292
354–284
[1303-86-2]
[10097-28-6]
[1310-53-8]
[1314-56-3]
[1314-62-1]
[1303-28-2]
[1314-60-9]
1820
602
606
1682-1326
6
2
2
4; 6
[13463-67-7]
[1314-13-2]
[1317-36-8]
[1344-28-1]
2159
1046
2029
498
8
4
6; 8
6
303
301
303
420–332;
280–221
269
261
338; 253
248
1514
1699
1669
1163
2159
970
929
602
606
602
1088
1075
1071
498
502
491
481
447
6
7
8
6
12
6
6
4
4
4
8
8
8
4; 6
6
9
10
12
252
242
208
193
179
161
154
150
151
150
135
134
133
124; 82
83
53
48
39
[12060-08-1]
[1312-81-8]
[1314-36-9]
[18282-10-5]
[1314-20-1]
[1309-60-0]
[1309-48-4]
[12057-24-8]
[1317-36-8]
[1314-13-2]
[1304-28-5]
[1305-78-8]
[1314-11-0]
[1306-19-0]
[1313-59-3]
[12136-45-7]
[18088-53-2]
[20281-00-9]
[1314-20-1]
[1304-56-9]
[1314-23-4]
[1306-19-0]
Vol. 12
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(NBO) bonds to modifying polyhedra (Fig. 5). The network modifiers are important constituents to most technological glasses because they lower the melting
temperature and control many useful properties. The intermediate oxides have
coordination numbers and bond strengths between the network formers and network modifiers and tend to have an intermediate effect on glass properties.
Silicate Glasses. The structure of silica glass consists of well-defined
SiO4 tetrahedra connected to another neighboring tetrahedron through each corner. Neutron diffraction studies indicate that the SiO distance in the tetrahedron is 0.161 nm and that the shortest OO distance is 0.263 nm, the same
dimensions as found in crystalline silica (9). The intertetrahedral (SiOSi)
bond angle distribution is centered near 1438, but is much broader than that
found for crystalline silica, producing the loss in long-range order shown schematically in Figure 4.
The structure of alkali silicate glasses also consists of a network of SiO4 tetrahedra, but some of the corners are now occupied by non-bridging oxygens that
are linked to the modifying polyhedra (Fig. 5). Increasing the concentration of
modifying oxide (R2O) increases the relative fraction of nonbridging oxygens
associated with the glass network and so reduces the Tg and melt viscosity
Fig. 5. Schematic 2D representation of the random network of alkali silicates (11).
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Fig. 6. Modified random structure showing ‘‘alkali channels’’ (14).
and increase thermal expansion coefficient and ionic conductivity. The changes
in the silicate network, and so the compositional dependence of many of the glass
properties, can be described by the relative fractions of bridging and nonbridging
oxygens or by the types and concentrations of the different Si-tetrahedra, viz, tetrahedra that possess four bridging oxygens (sometimes called Q4 tetrahedra),
those with three BOs (Q3), etc. On the atomic scale (0.1–5 nm), the distributions
of modifying alkali ions (Rþ) around bridging oxygens and nonbridging oxygens,
as well as the R–R distributions are not random (12,13). One view of the glass
structure is a ‘‘modified random network’’ in which the alkali ions and NBO cluster to form alkali-rich regions surrounded by presilicate network (Fig. 6). X-ray
and neutron diffraction studies, extended X-ray absorption fine structure
(EXAFS) data (14,15), and molecular dynamics (MD) simulations (16) give a picture of the glass structure consistent with that shown in Figure 6 (14).
Glasses containing < 10 mol% alkali oxides are considerably more difficult
to melt due to higher viscosities (17). Alkali-deficient glasses are prone to phase
separation and devitrification on a scale of 0.1–1 mm (18).
Borate Glasses. There are several reviews on the network structure of
borate glasses and alkali borate glasses (19–25). The structure of vitreous
B2O3 consists of planar triangular BO3 units that link to form larger units
known as boroxol rings (see Figure 7). These well-defined units are connected
by oxygens so that the BOB angle is variable and twisting out of the plane
of the boroxol group can occur, producing the loss of long range order associated
with glass. For vitreous boron trioxide (v-B2O3) the results by MD and quantum
mechanical simulations (21,26), nuclear magnetic resonance (nmr) (22), nuclear
quantum resonance (nqr) (23), ir and Raman spectroscopic (24) studies, and
inelastic neutron scattering (25) all indicate that a large fraction of B atoms
( 80–85%) are in the planar boroxol rings.
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Fig. 7. (a) Structure of crystalline B2O3 formed by BO3 triangles and (b) boroxol unit (20).
When an alkali oxide is first added to B2O3, the Tg increases and the coefficient of thermal expansion (CTE) decreases, and property trends are opposite to
those observed when modifying oxides are added to silica. This behavior is sometimes called the ‘‘Borate Anomaly’’ and it can be explained by the compositionally
dependent changes in the borate glass structure. For glasses with the molar composition xR2O(1 x)B2O3, the initial addition (up to x 0.25) of an alkali oxide to
B2O3 causes the trigonal borons (B3) associated with boroxol rings and ‘‘loose’’
BO3 triangles to convert to tetrahedral borons (B4) (27). Each of the four oxygens
associated with these new tetrahedral sites are bridging oxygens, and so the
changes in glass properties that define the borate anomaly can be explained by
the increasing number of structural linkages (through bridging BOB bonds)
with the initial addition of the modifying oxide. A maximum concentration of B4
sites are present when x 0.30 and further additions of the modifying oxide leads
to the replacement of the B4 units with B3 units that possess nonbridging oxygens (23,28). As a consequence, Tg decreases and thermal expansion coefficient
increases with additions of R2O beyond 30 mol%.
Phosphate Glasses. The basic building blocks of crystalline and amorphous phosphates are PO4-tetrahedra. These tetrahedra link through covalent
bridging oxygens to form various phosphate anions (see Fig. 8). The tetrahedra
O
O O
P O P O
O
O O–
P
O
P
Me
OO
–
O
P
O O
O O
P
O
O
P
O
O O
(a)
OO
P
O
O–
O
P
O
O–
P
O–
O–
O
O–
Me Me
O O
O–
O–
O
P O
P
P
O –O
O
–
O
O
O P
(b)
Fig. 8. Ultraphosphate glass structure in different concentrations of modifies ions (Me),
terminal oxygens > modifier ions (a) and terminal oxygens < modifier ions (b) (30).
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are classified using the Qn terminology (29), mentioned above, where n represents the number of bridging oxygens per tetrahedron. The networks of phosphate glasses can be classified by the oxygen/phosphorus ratio, which sets the
number of linkages though bridging oxygens to neighboring P-tetrahedra.
Thus, metaphosphate (O/P ¼ 3) and polyphosphate (O/P > 3) glasses have structures that are based on chain-like phosphate anions that are themselves interconnected though terminal oxygens by ionic bonds with modifying cations (30)
and ultraphosphate (O/P < 3) glasses have network that are cross-linked by Q3
tetrahedral with three BO and one double-bonded nonbridging oxygen to satisfy
the þ5 valence of the phosphorus (31).
The addition of an alkali or alkaline earth oxide to P2O5 depolymerizes the
three-dimensional (3D) Q3 network to form the chain-like Q2 sites. The resulting
depolymerization of the phosphate network with the addition of alkali oxide,
R2O, is described by the pseudo-reaction (32)
2 Qn þ R2 O ! 2 Qn1
ð3Þ
The extent of the network polymerization in silicate and phosphate glasses
changes monotonically as a function of composition, however, the compositional
dependence of a variety of ultraphosphate glass properties, are anomalous when
compared with the silicate analogues (33,34). For example, the minimum in density at 20 mol% Na2O (Fig. 9) is not consistent with simple network depolymerization and alkali packing (30,35).
In diffraction studies of binary ultraphosphate glasses, Hoppe and coworkers (36) described the role that the modifier coordination plays in determining the properties and structures of phosphates glasses. Hoppe assumed that
only nonbridging oxygens participate in the coordination shell of the modifier
cations. At low concentrations of modifier oxide, sufficient numbers of nonbridging oxygens are available to isolate the individual modifier polyhedra (Fig. 8(a))
but a higher concentrations, the increasing numbers of modifying polyhedra
must share available NBOs (Fig. 8(b)). The composition at which this change
in modifier packing occurs is dependent on the modifier coordination number and
2.6
Density (g/cm3)
2.55
2.5
2.45
2.4
2.35
2.3
2.25
0
Fig. 9.
10
20
30
40
mol% Na2O
50
60
Density of sodium ultraphosphate glasses (30). r Na-phosphates j Na-silicates
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can be related to the changes in glass properties. Thus, the phosphates are an
example of a glass-forming system in which a detailed understanding of the
structural roles played by both the glass forming and modifying constituents
must be obtained to properly explain composition–property relationships.
Germanate Glasses. The structure of GeO2 glass is very similar to that
of SiO2 glass, with basic building block of germanium–oxygen Q4 units (37).
Since the Ge4þ ion is larger in diameter than the Si4þ ion, the GeO distance
is also larger than the SiO distance (in silicate glasses), with a bond length
of 0.173 nm and GeOGe bond angle smaller than the SiOSi bond angle.
Gas diffusion studies suggest that the open volume of germanate glass is slightly
less than that of silicate glass (38). Recent reports including neutron diffraction
(39), high energy photon diffraction (using synchrotron radiation) (40), magic
angle spinning nuclear resonance (mas nmr) (41) and Raman spectroscopy (42),
suggest that the structure of vitreous germania resembles that of quartz-like
GeO2, with [GeO4] tetrahedra providing the basic structural units, giving a continuous random network. Jain and co- workers (43) have used xps (x-ray photoelectron spectroscopy) to investigate the effect of alkali additions to germania,
showing, in contrast to previous models, that nonbridging oxygens are formed
at very low alkali concentrations (2%) along with GeO6 units.
Chalcogenide Glasses. Chalcogenide glasses are produced by melting
group 16 (VI A) elements (S, Se, and Te) with other elements, generally of
group 15 (V A) (eg, Sb, As) and group 14 (IV A) (eg, Ge, Si) to form covalently
bonded solids. When melted in an atmosphere particularly deficient in oxygen
and water, the glasses have unique optical and semiconducting properties (44).
Structural models for these glasses are based on the high degree of covalent
bonding between chalcogenide atoms. Since the chalcogenide glasses are a set
of continuously varying compositions of elements having a varying covalent coordination number, it is generally useful to invoke the concept of the atomaveraged covalent coordination, hri, as a structural attribute
hri ¼ ri ai
ði ¼ 1; 2; :::; nÞ;
ð4Þ
where ri is the covalent coordination number of element i having atom fraction ai
in the glass. Thorpe (45) suggested that glasses having hri < 2.4 possess structures with regions whose volume fractions are too small to be fully connected.
This lack of full connectivity results in a ‘‘polymeric’’ solid where the rigid regions
are surrounded by a ‘‘floppy’’ matrix. When, hri > 2.4, the solid has continuously
connected rigid regions with floppy regions interdispersed and may be termed an
‘‘amorphous solid’’. The hri ¼ 2.4 glass is unique in that it has the number of constrains equal to the number of degrees of freedom, consisting of floppy and rigid
regions individually connected by matrices with maximum number of connections. There are a number of studies that relate the degree of structural connectivity to the glass-forming tendency and properties of chalogenide glasses eg.
(46–48).
Halide Glasses. Structural models for fluoride glasses based on BeF2 are
directly analogous to those for alkali silicate glasses, with the replacement of
nonbridging oxygens by nonbridging fluorines (NBF). Fluoride glasses have
been studied for the past 30 years and have found various applications in optics
576
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(48), sensors (49), is instrumentation, medicine and telecommunications (50). Of
particular importance are the heavy metal fluoride glasses (HMFG) based on
ZrF4 in numerous multicomponent systems in which some fluorides act as
glass formers in association with alkali and divalent fluorides (51). Extensive
development work has also been carried out on fluorophosphate glass (5–20%
P2O5), initially for use as optical glasses but more recently for use in high
power lasers (52).
Organic Glasses. Organic glasses consist of carbon–carbon chains,
which are so entangled, that rapid cooling of the melt prevents reorientation
into crystalline regions. Like low crystallinity glass–ceramics, the organic
glasses presented small regions of oriented chains (53). Low molecular weight
organic glasses are increasingly investigated because they potentially combine
several interesting properties such as easy purification, good processability
and high gas solubility (54). Numerous applications are envisaged, eg, in light
emitting devices (55), in nonlinear optics (56), in optical data storage (57), and
in photovoltaic and photochromic materials (58). Consequently, the influence of
the molecular structure on stability of the glass and on the Tg is an important
question.
Metallic Glasses. Structural models for metallic glasses include variations of the random network theory, crystalline theory, and a dense random
packing of spheres. Structural methods such as X-ray diffraction (59), electron
microscopy (60), Mössbauer resonance, nmr, and thermal analysis (61), have
been used to study the structures of glassy metals. Heat capacity data demonstrated that the metals were indeed vitreous and not amorphous with microcrystallization.
Metallic glasses were first produced commercially as ribbons or fibers
50–100 mm thick and up to 25 mm wide. For example, bulk glassy alloys in
the Mg (62), La- and Zr- (63) based system, having a large supercooled liquid
region before crystallization, have attracted much interest as new materials in
science and engineering fields (64). The glass-formation ability of a melt is evaluated in terms of the critical cooling rate, Rc, for glass formation. The critical
cooling rate is the minimum cooling rate necessary to keep the melt amorphous
without precipitation of crystals during the solidification and is shown schematically in Figure 3. There are now some compositions, with lower Rc, that can be
cast as monoliths. For example, metallic glasses such as Au77.8Si8.4 Ge13.8 and
Fe91B9 have Rc of 3 106 and 2.6 107 K/s (60), respectively, whereas more
recent bulk metallic glasses based on alloys of Zr, Ti, Cu, Ni, and Be have critical
cooling rates on the order of 10 K/s (65,66).
Many studies on the formation and structure (ie, Refs. 67–69), physical and
mechanical properties, and Tg and crystallization process of glassy alloys have
been reported (ie, Refs. 70,71). Examples include Fe-based bulk metallic glasses,
which have been prepared in Fe–(Al, Ga)–(P, C, B, Si) (72), Fe–(Co,Ni)–(Zn, Nb,
Ta)(73), and Fe–C–Si–B, Fe–Ni–P–B system (74). They exhibit high glassforming ability, good mechanical properties, and soft magnetic properties. However, there are few results about corrosion resistance of iron-based bulk glassy.
2.3. Computer Modeling of the Glass Structure. Recent software
and hardware developments have produced a new characterization technique
for glass structure: atomistic simulations based on MD calculations of silicates
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577
(75,76), borate glasses (77,78), and phosphate glasses (79). Static lattice simulations cannot be applied in a straightforward way to glasses as in the study of physics and chemistry of crystalline solids. The MD studies of alkali silicates (Na–,
K–, Na–K, and Li) provide ‘‘snapshot’’ pictures of the atomic configuration. This
allows the identification of key features and correlation of the atomic scale structure with the macroscopic experimental properties.
The distribution of alkali modifiers throughout the glass network is one
aspect of technological importance. Studies of alkali silicate glasses reveal that
the alkali ions are not randomly distributed within the silica network but rather
aggregate in alkali-rich regions on a nanoscale, consistent with the ‘‘modified
random network’’ structural model introduced in the section on Silicate Glasses
(75). Lithium-silicates exhibit the greatest degree of aggregation, possibly
because of the size and mobility of the ion. The disilicate composition marks
the onset of the thermodynamically predicted homogeneous glass-forming
region. Such results help relate phase separation and immiscibility tendencies
for the alkali silicates to structural and thermodynamic considerations.
Surfaces can be modeled using MD in two ways (77): by removing the periodicity in one dimension or by increasing the dimension of one of the box edges,
without scaling the atomic coordinates. The second method creates a series of 2D
slabs with top and bottom surfaces. Figure 10 shows the vitreous silica surface
computer simulations obtained at the New York State College of Ceramics at
Alfred University. Such calculations give new insight on the glass structure.
Direct views of the structure of a silica glass fracture surface and comparison
with a structure calculated by MD simulation of SiO2 glass surface provides support for Zachariasen’s random network structure model of glass (80).
2.4. Glass Ceramics. Glass–ceramics are normally obtained by a controlled crystallization process of suitable glass-forming melts. Internal or external nucleation is promoted to develop microheterogeneities from which
crystallization can subsequently begin. As a result, the amorphous matrix
transforms into a microcrystalline ceramic aggregate. The composition of the
Fig. 10. (a) Top view of vitreous silica surface with an area of 2.83 2.83 nm2. All species
>2.9 nm are shown: threefold Si (turquoise triangle), fourfold Si (yellow polyhedron),
fivefold Si (purple polyhedron), NBO (purple sphere), BO (blue sphere), and TBO (terminal BO, red sphere); (b) Top view of 30% sodium silicate glass surface with an area of
2.732 2.732 nm2, showing all species >2.7 nm (75).
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crystalline phases and the crystalline sizes define the properties of the final
material. Therefore, the major components and the composition of the glass
are selected to ensure precipitation of crystals that provide desired properties
of the glass–ceramic (81–83). By definition, glass–ceramics are > 50% crystalline after heat treatment; frequently, the final product is > 90% crystalline. In
general, the heat treatment necessary to convert the base glass into a glass ceramic increases the fabrication costs of a component. Consequently, many interesting glass ceramics have been developed, but not all of then have been
commercially successful, due to the ratio between customer benefit and unattractive price (84).
Aluminosilicate glass–ceramics are among the most useful commercial products. The addition of nucleation agents such as TiO2 or TiO2 þ ZrO2 and the
selection of the optimum heat treatment schedule controls the distribution and
morphology of the final crystal structures. Many other components can be added
to optimize the crystalline phases and the glass–ceramic properties. Certain
lithium aluminosilicates have low expansion and good chemical durability.
Sodium aluminosilicates and barium–sodium aluminosilicates have high expansion and can be strengthened by surface compression techniques such as the
application of a low expansion glaze. Magnesium aluminosilicates have low
expansion and can have very high strength. Fluorine added to potassium–
magnesium aluminosilicate increases machinability (85).
Other systems preferentially crystallize at surfaces, thus glass powders can
be converted to glass–ceramics without the need for the addition of special nucleating agents. The densification of the glass-powder compact must take place
prior to crystallization. During the sintering stage, the glass grains first densify
by viscous flow and then nucleate at and crystallize from the original glassparticle boundaries. Surface nucleation is very important for many applications
of sintered glass–ceramics and in most cases, surface crystallization is delayed
until densification has proceeded. Table 3 shows examples of commercial glass–
ceramic systems.
2.5. Devitrification and Phase Separation. Devitrification is the
uncontrolled formation of crystals in glass during melting, forming, or secondary
processing in contrast to the controlled crystallization associated with glass–
ceramic processing. Devitrification can affect glass properties including optical
transparency, mechanical strength, and sometimes the chemical durability. As
discussed in the section Kinetic Theory of Glass formation, glass-formation ability (GFA) depends on the avoidance of devitrification. The GFA of a melt is evaluated in terms of the critical cooling rate Rc, for glass formation, which is defined
as the minimum cooling rate necessary avoid precipitation of any detectable crystals during solidification. Systems with lower Rc (line B in Fig. 3) have greater
GFA. The supercooled liquid temperature DTxg (the temperature difference
between the onset crystallization temperature Tx and the Tg), is another indication of the devitrification tendency of a glass upon heating above Tg. A large DTxg
value indicates that the supercooled liquid can exist in a wide temperature range
without crystallization and has a high resistance to devitrification (87).
Glasses that derive their color, optical transparency, or chemical durability
from a small amount of a finely dispersed, amorphous, second phase are termed
phase-separated glass, distinguished from glass ceramics because they remain
predominantly amorphous. Phase separation can occur by processes: (1) nuclea-
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Table 3. Commercial Glass–Ceramicsa
Commercial
designation
Major crystalline
phases
Corning 9632
b-quartz solid
[14808-60-7]
Corning 9608
b-spodumene solid
solution, Li2O.
Al2O3 (SiO2)4
b-spodumene
[1302-37-0]
nepheline [12251-37-3],
Na2O Al2O3 2SiO2
a-quartz solid solution
(SiO2); spinel
(MgO Al2O3);
enstatite MgO SiO2
3Al2O3 2SiO2;
(Ba, Sr, Pb) Nb2O6
b-spodumene solid
solution; mullite
[1302-93-8],
3Al2O3 2SiO2
Neoceram
(Japan)
Corning 0303
Corning 9625
High K
(Corning)
Corning 9455
Properties
Application
low expansion, high
strength, thermal
stability
low expansion,
high chemical
durability
low expansion
electrical range tops
high strength, bright
white
very high strength
high dielectric
constant
low expansion, high
thermal and
mechanical
stability
cooking utensils
cooking ware
tableware
classified
capacitors
heat exchangers
a
Ref. 86.
tion and growth and (2) spinodal decomposition (88). The morphologies of the
phase-separation microstructures obtained by these two different processes are
different. Spinodal decomposition produces a composite material with two highly
interconnected amorphous phases, whereas phase separation that occurs by the
classical nucleation process produces a microstructure in which discrete, spherical droplets are embedded in an amorphous matrix (89). The most important
parameter affecting the morphology of phase separation is the composition of
the liquid. Discrete particle morphologies will be observed for compositions
near the edges of liquid–liquid miscibility gaps. Morphologies with larger
volume fractions of both phases, often with a greater degree of connectivity,
will be found for composition near the center of miscibility gaps. Figure 11 summarizes and exemplifies nucleated phase separation and spinodal decomposition.
Fig. 11. Microstructural morphology of immiscibility in glasses: (a) sodium-borosilicate
glass showing a nucleated type of phase separation, and (b) sodium-borosilicate glass composition from center of immiscibility region (spinodal decomposition) (90).
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2.6. Surfaces of Glasses. The surface of a glass plays a major role in
its ability to function in a given application. For example, optical applications
may require smooth glass surfaces to precise dimensions (ie, lenses) and high
chemical durability. In other applications, the surface must form appropriate
bonds to specific materials (decorations, coatings). Four characteristics of the
surface make a glass suitable for particular applications: (1) ability to be ground
and polished, (2) chemical durability, (3) ability to bond specific molecules, and
(4) resistance to mechanical damage (strength is limited by presence of Griffith
flaws). Table 4 summarizes selected tools and techniques for the study of glass
surfaces (91,92).
Fiber surface characteristics determine most of the important properties of
continuous glass fibers used for composite reinforcements. Applications of coatings (sizing) agents serve many purposes, including process compatibility,
scratch protection, chemical passivation, and adhesion promotion.
The use of glass as a substrate for flat-panel displays (FPD) exerts considerable demands on the glass surface. It must be smooth, free of particulate contamination and capable of interfacing with metals, semiconductors, oxides, and
polymers. In the manufacture of FPD devices, the surface must withstand the
chemical and physical processes associated with wet and dry cleaning, chemical
etching, polishing, and plasma treatments.
3. Properties
The properties of glasses depend on their chemical composition and their structure. Most properties can be discussed from a starting point represented by the
material of the crystalline form by considering what modifications, structural
disorder, absence of translation periodicity, spatial variations in atomic concentrations or local structure will have on the chosen property (93). A major advantage of glasses is that their properties can be tailored by adjusting their
composition. As a first approximation, a final given property can be expressed
as a simple additive function of its relative oxide contents. However, in some
cases the relationship is more complex (ie, borate anomaly). Several compilations
of experimental results are useful sources of data (94,95). More recently, such
compilations are maintained electronically. For example, Sciglass (96) is a database that includes >1,000,000 experimental values for 105,000 glasses, >60% of
the world’s published glass data. Interglad (International Glass Database
System) is another commercial electonic system compiling data on >190,000
glasses of different compositions (97).
Another compilation is being generated by the National Science Foundation
(NSF)–Industry University Center for Glass Research (CGR), where researchers
at Alfred University and at the Thermex Company in St. Petersburg, Russia, are
developing a glass melt property database for the glass manufacturers who
model glass melting and forming processes. The compositions being studied comprise six types of glass: container glass, float glass, fiberglass (E and wool types),
low expansion borosilicate glass, and TV panel glass. The melt properties include
gas solubility, density, thermal expansion, surface tension, viscosity (Newtonian
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Table 4. Some Current Techniques for Studying Glass Surfacesa
Technique
Comments/general information
Characteristics measured
nuclear reaction
analysis
detection of g-radiation from the
nuclear reaction of 15N and H
quantitative technique;
analyzes light elements,
including hydrogen, and
concentration depth profile
in hydration processes
density of the surface or films;
surface and buried interface roughness; film thickness; distinction between
physical roughness and
chemical gradients at
interfaces; chemical
composition; and elementspecific coordination
number, bond distances,
and oxidation states
analyzes modifications of the
surface composition (aging,
dealkalization, diffusion)
and layers deposited on
the surface (organic and
inorganic) with lateral
resolution of 0.1–1 mm
monitors the interaction of
organic and inorganic
coatings with inorganic
glass substrates
X-Ray Scattering
secondary ion mass
spectroscopy
SIMS can perform depth profiles
with in-depth resolution of
0.2–5 nm range
fourier transform
infrared
spectroscopy
other FTIR spectral methods:
DRIFT (diffuse reflectance
infrared fourier transform), ATR
(attenuated total reflection), and
PA (photo-acoustic spectroscopy)
AFM probes the surface with a tip,
2 mm long and 100Å diameter,
located at the free end of a cantilever 100–200 mm long. Forces
between tip and the surface
cause the cantilever to deflect
which is measured as the tip is
scanned over the sample and a
computer generates the surface
topography
EPMA uses incident electrons to
excite the glass surface with
characteristic X-rays emitted
atomic force
microscopy
electron probe
microanalysis
X-ray photoelectron
spectroscopy
microscope and high precision
2D profilometer, lateral
resolution > 0.1 nm and
height resolution > 0.01 nm
(glass structure, roughness
and surface defects,
corrosion and aging,
fracture mechanics, and
coatings on glass)
chemical information at or
near glass surface; WDS
(wavelength dispersive
spectrometry) detects light
elements
outermost composition
5–10 nm
a
Refs. 91,92.
and non-Newtonian), heat capacity, and radiative thermal conductivity (98). Full
review of the model and database by selected CGR glass companies is expected by
December 2003.
3.1. Optical Properties. Probably the most striking characteristic of
conventional glasses is their transparency to visible light resulting from the
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Fig. 12.
Light guiding in a clad fiber (99).
absence of grain boundaries and light-scattering defects. The optical transparency of high purity silica glass made it possible to develop efficient optical fibers
and devices. For practical light-transmitting fibers, a cladding glass with a lower
refractive index, n, surrounds the core glass and light is guided through the core
by internal reflection on the interface between the core and cladding. The difference in n between core and cladding determines the acceptance angle (or numerical aperture) for incoming light (see Fig. 12).
Optical glasses are usually described in terms of their refractive index at
the sodium D line (589.3 nm), nD, and their Abbé number, n, which is a measure
of the dispersion or the variation of index with wavelength. Glasses with nD <
1.60 and n < 55 are defined as crown glasses and those with nD > 1.60 and n <
50 are defined as flint glasses (Fig. 13). A low dispersion is desirable in optical
glasses used for lenses because dispersion causes chromatic aberration. Fluorophosphates, having absorption edges located well into the ultraviolet (uv), are
examples of glasses with high Abbé numbers and low refractive indexes.
The loosely bound valence electrons make the greatest contribution to n, so
large ions, such as Pb(II) or Bi(III), are added to glass to increase the refractive
index. Glasses from the PbOBi2O3Ga2O3 have refractive indexes for visible
wavelengths as high as 2.7. Other high index commercial glasses have 30–70%
TiO2, 10–50% BaO and 0–10% ZrO2 (wt%) plus small amounts of other oxides. Such compositions require high melting temperatures, 15008C and above,
which together with their high chemical corrosiveness toward refractories is a
severe limitation to preparing these glasses by conventional melting methods.
The distinguishing features of borate glasses, relatively high refractive index
and low dispersion, are related to the large number of molecules in a unit
volume, N, compared with those of the other glasses (100).
The addition of alkali or alkaline-earth oxides to a glass-forming oxide
shifts the uv absorption edge to lower energies (longer wavelengths). Conversely,
the range of uv transmission is enhanced when the cations in the glass have a
high charge/radius ratio, indicating a stronger cation–oxygen bond. High purity
fused SiO2 glass has been developed that is highly resistant to optical damage by
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Fig. 13. Index of refraction vs dispersion and optical classification of glasses. The shaded
area indicates region of glass formation. BaF ¼ barium flint; BaK ¼ barium crown;
BaLF ¼ light barium flint; BaSF ¼ heavy barium flint; BK ¼ borosilicate crown; F ¼ flint;
FK ¼ fluorcrown; K ¼ crown; KF ¼ crown flint; LaF ¼ lanthanum flint; LaSF ¼ heavy
lanthanum flint; LaK ¼ lanthanum crown; LF ¼ light flint; LLF ¼ very light flint; PK ¼
phosphate crown; PSK ¼ heavy phosphate crown; SF ¼ heavy flint; SK ¼ heavy crown;
SSK ¼ very heavy crown; TiF ¼ titanium flint.
uv (190–300-nm) radiation. The glass exhibits no optical damage after 107
pulses (350 mJ/cm2) from KrF lasers at 248 nm and from ArF lasers at 193 nm
(101). The addition of nitride ions to oxide glasses shifts the uv edge to longer
wavelengths, probably because of the greater polarizability of the trivalent nitrogen. Nitride glasses, in contrast to conventional optical glasses, or fluoride optical glasses, posses a remarkable combination of desirable properties, including,
high hardness, high refractive index, and high softening temperature (102).
In the visible region, absorption by additives such as transition metal or
lanthanide ions is usually more important than contributions from the glass formers themselves. Several references discuss in detail the generation of color in
glass (eg, Refs. 103–105). The coloration of glass by uv radiation from sunlight
(solarization) results from the oxidation of transition metal ions in the glass.
Optically pumped laser action has been observed for most lanthanide ions in
a variety of glass systems. Large, high power neodymium glass lasers have
been used for inertial confinement fusion experiments. The best glass laser
systems have the following qualities: the absorption spectrum of the lasing ion
584
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matches the spectrum of the pump radiation; the absorbed radiation efficiently
produces excited-state ions; the excited state has a long lifetime; the probability
of radiative decay is high; and the line width of the emitted radiation (fluorescence) is narrow. The line width of the fluorescence band of the lanthanide ion
is affected by the glass matrix. In general, the smaller the field strength of the
anions, the less the perturbation of the coordination shells of the fluorescing ion
and the narrower the line width, ie, fluoride and chloride glasses promote narrower line widths than those seen in oxide glasses (106).
The visible transmission of photochromic glasses decreases with increasing
frequency of light, and the effect is reversible. These glasses contain 10-nm
droplets of silver chloride, AgCl, or other silver halides doped with copper(I)
ions. In the presence of uv radiation, the reaction Ag(I) þ Cu(I) ! Ag þ Cu(II)
occurs, leading to the formation of small particles of silver causing the glass to
darken (107).
Chalcogenide glasses such as As2S3 are colored or even opaque, because of
the small difference in energy between the conduction and valence bands. On the
other hand, color in reduced amber glasses is the result of a Fe3þS2 chromophore, not involving Fe2þ (108).
3.2. Chemical Durability. The chemical durability of glass is critical for
many applications, including the performance of glass containers for food and
beverages, pharmaceuticals, and corrosive chemicals; the retention of high
transparency for optical components, including windows, exposed to ambient
conditions; the use of glass as a long-term host for radioactive and hazardous
materials; and the performance of bioactive glasses implanted in the body.
Numerous reviews exist that describe the chemical interactions between glass
and various environments (eg, Refs. 109–111).
Silicate Glasses. The leaching of alkali-containing silicate and borosilicate glasses in aqueous solutions is considered as two processes occurring in parallel: exchange of alkali ions for H3Oþ from the solution (controlled by diffusion
of ions through a hydrated layer) and dissolution of the hydrated layer (controlled by surface reaction kinetics). The chemical durability of glass against
reactions with aqueous solutions is determined by sample states and by corrosion
conditions. Sample states include glass composition, mole fraction of crystalline
phases, internal or applied stresses, surface roughness, phase separation, and
homogeneity of powder or bulk for of the material. Corrosion conditions include
relative humidity, gas surface reactants, pH of solution, initial and final composition of corroding solution, pressure and temperature of the system, and ratio of
the corroded area to the volume of the corroding medium (112). As a first approximation, the durabilities of alkali silicate and alkali borate glasses in aqueous
solutions can be estimated from thermodynamic calculations (113,114). This
approach is useful for describing the major species in solution and has established qualitatively that (1) alkali silicate glasses are less durable than silica,
(2) the solubility of alkali silicate glasses increases with increasing pH, and (3)
the relative stability of alkali silicate and alkali borate glasses should increase
in the order of modifier oxide as K2O < Na2O < Li2O; as observed experimentally.
However, such calculations neglect kinetic processes such as formation of diffusive layers and reprecipitation of glass constituents, as well as structural
features of the glass network.
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Certain network modifiers and intermediate oxides (Table 2) reduce substantially the rate of attack on alkali silicate glasses (eg, alkaline earth ions,
Zn2þ, Al3þ, Zr4þ). Alkaline earth ions promote the formation stable leached
layers, whereas Al3þ, Zr4þ increase the thermodynamic stability of the glass.
The outstanding chemical durability of borosilicate glasses, like Pyrex, in aqueous solutions is a result, in part, of phase-separated structures. The durability
is dependent on the amount of network modifier, the amount of B2O3, and the
thermal history of the glass.
The resistance of silica and silicate glasses to sodium vapor (as in the use of
highway sodium lamps) has been studied by several authors (115). The attack of
silicates by sodium vapor (as example of attack by alkaline vapors) is explained
by diffusion of sodium into the glass and then reaction between sodium and the
glass.
Borate Glasses. Boron oxide is highly soluble in water and borate glasses
are very hygroscopic. The addition of alkali oxide increases the number of fourcoordinated boron tetrahedra (up to 30 mol% alkali oxide), which strengthens
the structure and increases the resistance to chemical attack. Further addition of
alkali oxide produces nonbridging oxygens, decreasing the resistance to aqueous
dissolution. Alkali silicate glasses usually dissolve in aqueous solutions following
a diffusion process (t1/2 law, pH<9, sufficiently short times) whereas alkali borate
glasses display linear kinetics of dissolution (116). The basic difference is the
ability of silicate glasses to form a diffusive layer for the transport of alkali
ions which in turn controls the overall process.
Fritted glasses have become the common method of incorporating borates
into glaze and vitreous enamels. Major benefits of borate use include reducing
thermal expansion and improving durability of the glaze. Borate sources are
mainly borax and colemanite, and common commercial forms (minerals; refined
minerals; and synthetic compounds) such as boric acid and borax pentahydrate
(117).
Phosphate Glasses. Many phosphate glasses have a chemical durability
inferior to that of most silicate and borosilicate glasses. Metaphosphate glasses
are most common and the metal ions that link neighboring phosphate anions are
readily hydrated, causing the entire phosphate chain to be released into the aqueous environment (118). Iron phosphate glasses are an exception (119–121).
Because of their unusually high chemical durability, iron phosphate glasses
and zinc–iron phosphate glasses are of interest for nuclear waste immobilization. The most durable compositions have O/P ratios near 3.5 and so are considered pyrophosphate compositions. Additionally, iron phosphate glasses have
low melting temperature, typically between 950 and 11508C. Investigations of
iron phosphate wasteforms obtained by adding different amounts of various
simulated nuclear wastes to a base iron phosphate glass, 40Fe2O360P2O5,
showed that these glassy wasteforms have a corrosion rate 100 times lower
than a typical sodalime silicate glass. Generally, iron phosphate glasses can contain up to 40 wt% of certain simulated waste (122).
3.3. Electrical Properties. Ion Conducting Glasses. In alkali containing glasses, charge is carried by alkali ions moving from modifier site to
modifier site, and so properties like conductivity are sensitive to composition
(ie, the number of charge carrying ions) and structure (the nature of the modifier
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site). Glasses with very high conductivities have been developed as electrolytes
for solid-state batteries (123, 124). Superionic conducting glass systems include
(for Agþ) AgIAg2OMoO3, AgIAg2OP2O5, AgIAg2OB2O3; (for Liþ)
Li4SiO4Li3BO3, LiClLi2OB2 O3; (for Cuþ) CuICu2OP2O5, CuICu2O
MoO3; and (for Naþ) Na2OZrO2P2O5SiO2 (125). For example, the Naþ
ionic conductivities of the glass–ceramic Na4.1Sm0.5P0.4Si2.6O9 and Na4.1Y0.25
P0.4Si2.6O9 are reported as 4.78 102 and 2.79 102 S/cm at 3008C, respectively (126,127). Glasses and glass–ceramics that do not contain alkali oxides
have low bulk electrical conductivities under normal conditions that increase
somewhat with temperature; such materials are used as high temperature insulators in electrical and radio engineering (128).
Electrical conductivity of glasses in the system Li2Cl2Li2OB2O3 has been
measured by the complex impedance method at 100–20,000 Hz using threeelectrode connection of the specimen in the circuit. There is a distinct increase
in conductivity and decrease of the activation energy with increasing content
of Li2Cl2. Increased content of Li2O brings about a mild increase in conductivity
and a mild decrease of activation energy (129). Electrical conductivity, Raman
spectra, and the glass-forming region have been determined in borate glasses
containing lithium sulfate. The relation between conductivity and composition
is discussed with reference to the glass structure (130).
Anomalies in the (x)AgI(1-x)AgPO3 glasses (x0.3) are observed in the electric properties, molar volume, and also local probes like the 31P nmr relaxation
time. These anomalies can be explained in terms of the opening of percolative
channels among the metaphosphate chains, which are subsequently filled by
the dissociated Ag+ and I ions. An attempt to reconcile the different data on activation energy for dc-conductivity and Tg reported in the literature has been made
(131).
Protonic conduction in 10P2O5 90SiO2 and 20P2O5 80SiO2 (mol%) glasses
prepared by sol–gel processing have been investigated as a function of the content of molecular water adsorbed in the pores. The results show that the electrical conductivity varies exponentially with the reciprocal absolute temperature
and increases with the increase of the content of the adsorbed molecular
water. The double-bonded oxygen and the high affinity of phosphorus for oxygen
make protons easy to release and transfer, which is favorable to the protonic conductivity (132). The Cuþ conducting glass–ceramics, in particular CuTi2(PO4)3
based materials having the Nasicon structure, have been described and use as
Cuþ ion conductors for low temperature O2 sensors (133). The partial substitution of Zn2þ for Agþ in Ag4P2O7 leads to the formation of a wide glassy domain of
composition (Ag4P2O7) (1y)(Zn2P2O7) (y ¼ 0.20–0.87) (134).
Mixed-Alkali Effect. In single alkali glass systems, different processes
contribute to the electrical conduction at different temperature. In general, the
ionic conduction is due to the motion of alkali ions and as a consequence, the electrical conductivity is expected to be proportional to the concentration of the alkali
ions.
The substitution of a second alkali ion, at constant alkali content, in many
phosphate, borate, and silicate glasses causes a decrease in the electrical conductivity up to five orders of magnitude. This is called the mixed-alkali effect (MAE),
observed in ionic conductive glasses (135,136).
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Table 5. Electrical Conductivity at Room Temperature (s300)
for Some Phosphate Glassesa
Glass composition
80Fe2O320P2O5
V2O5P2O5
50Fe2O350P2O5
20Fe2O330CaO50P2O5
50V2O5 60P2O5
40Fe2O360P2O5
Fe2O3TeO2P2O5
Li2OB2O3P2O5
(40x)Fe2O3. xNa2O.60P2O5
24Cs2O26.8Fe2O349.5P2O5
20Fe2O320K2O60P2O5
30Fe2O39Na2O61P2O5
a
s300 (O1 cm1)
3 1010
105
1010
1012
4.6 1010
8 1012
108 –1014
107 –106
1013 –1010
8.5 1011
3.0 1011
8.9 1012
Ref. 138.
As described by Day (135), the lower conductivity of mixed-alkali glasses
has been attributed to changes in both the size of the alkali ions and to an interaction between different alkali ions and the glass network. The diffusion with
memory model proposed by Bunde and co-workers (137) reproduces the variations of sdc as well as sac conductivity in mixed-alkali (Na, K) silicate gasses.
The studies of the alkali mixed effect have been related to the ionic conductivity
in alkali silicate, phosphate, and borate glasses (Table 5) and there are little
works on electronic conductive glass such as the studies in iron-phosphate
glasses.
Semiconductimg Glasses. Amorphous selenium and other chalcogenide
glasses form the basis for the multibillion dollar electrostatic copying industry.
Chalcogenide glasses can be switched between low and high conductivity states
using an applied voltage. There are two types of switching: threshold and memory. In the case of threshold switches, a small current is required to maintain the
ON (high conductivity) state. In contrast, memory switches remain on indefinitely in the absence of a current and require a short, high current pulse to
return to the state. A typical glass for a memory switch contains Ge, Te, and
either As, S, or Sb. The ON state in threshold switching is thought to arise
from the saturation of charged defect centers.
Semiconductivity in oxide glasses involves polarons (conducting electrons in
an ionic solid together with the induced polarization of the surrounding lattice).
In oxide glasses the polarons are localized, because of substantial electrostatic
interactions between the electrons and the lattice. Conduction is assisted by
electron–phonon coupling, ie, the lattice vibrations help transfer the charge carriers from one site to another. Cations capable of multiple valences facilitate
small-polaron conductivity. Vanadium and tungsten ions readily assume multiple valences, and vanadium oxide and tungsten oxide glasses exhibit some of the
highest electrical conductivities of any oxide glass. Phosphate and tellurate(IV)
glasses containing substantial amounts of multiple-valent transition-metal ions
such as iron or copper are also semiconducting.
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3.4. Thermal Properties. When a typical liquid is cooled, its volume
decreases slowly until it reaches the melting point, Tm, where the volume
decreases abruptly as the liquid is transformed into a crystalline solid. This phenomenon is illustrated by line A in Figure 1. If a glass-forming liquid is cooled
below Tm (line B in Fig. 1) without the occurrence of crystallization, it is considered to be a supercooled liquid until the Tg is reached. At temperatures below Tg,
the material is a solid. Faster cooling yields a less dense glass, as shown by line C.
Unlike the abrupt melting of a crystalline solid, the Tg is characterized by a
continuous change in properties over a small temperature interval. When a solid
glass is heated from below Tg, the volume and specific heat increase. As the Tg is
reached, the rates of change of these quantities become greater, indicating that
bonds are being broken and that some parts of the glass have become more
mobile; ie, above Tg the behavior of the glass becomes more like that of the liquid
phase.
The Tg of silicate glasses usually decreases as modifying oxides such as
Na2O are added because of the formation of nonbridging oxygen atoms. Although
Tg is important regarding glass formation, other temperatures are more useful
from a technological point of view. For example, the American Society for Testing
and Materials (ASTM) (139) defines several characteristic temperatures in terms
of viscosity (Fig. 14; working point (viscosity of 103Pas), softening point
(106.6 Pas), annealing point (1012 Pas), and strain point (1013.5 Pas) (140–
142). The annealing point temperature is close to Tg, at which temperature the
glass structure (and stresses) will relax in minutes. If annealing is carried out at
the streain point, the reduction of stresses to acceptable levels takes 4 h.
The temperature dependence of the viscosity of a glass melt is n onArrhenian a rid is often described by the Vogel-Fulcher-Tamman (VFT) equation
¼ 0 exp
B
T T0
ð5Þ
where 0, B, and T0 are fitting parameters.
High silica glasses such as Pyrex have low CTE (coefficient of thermal
expansion) and are used in applications requiring good resistance to thermal
shock. Ultralow expansion SiO2TiO2 glasses have CTEs of practically zero, as
do certain lithium-aluminosilicate glass ceramics, like Zerodur. Some applications, such as glass-to-metal seals, require glasses to have higher CTEs to
match metals and other materials. Highly modified silicate glasses and glass–
ceramics and phosphate glasses have been developed for high CTE
(>10 106/8C) sealing applications.
The thermal conductivity of glass is dependent on lack of long-range structural order. The mean free path of a phonon in a glass is on the order of a few
interatomic spacings, so phonons are damped out over very short distances, making glasses good thermal insulators, at least up to temperatures where radiative
processes become dominant. Thermal conductivity increases when glasses are
crystallized to form a glass–ceramic. On the other hand, the thermal conductivity of an aerogel is exceptionally low. Recent developments have combined this
property of silica aerogels with polymer cross-linking to develop very high
strength and very light materials, for potential applications in aerospace (143).
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1014
1013
1012
Positive
upper
use
temperature
Approximate
strain
Setting
point
temperature
for glass–to–metal
seals
Approximate
annealing
point
General
annealing
range
1011
1010
589
Chain
marking
in lehr
Glass will
deform under
gravity
Tubing
collapsed
under
vacuum
Viscosity Pa’1
109
108
107
Approximate
softening
point
106
105
104
103
102
Thermal
repressing
Bottle
gob
feeding
Approx.
working
point
Hot
sever or shear
Working
end of
tank
Vello
bowl
Downdraw
Turret
chain
Updraw
Danner
machine
lip
Melting end
of tank
10
Fig. 14.
Viscosity range of glass with relation to main processes.
3.5. Mechanical Properties. High strength glass fibers combine high
temperature durability, stability, transparency, and resilience at low cost
weight–performance. Various glass compositions have been developed to provide
combinations of fiber properties for specific end-use applications. Tables 6 and 7
provide information on selected compositions. The mechanical properties of silica
optical fibers have been studied extensively in recent years because of their use
in optical technologies such as lightguides and in high energy laser applications
(144,145).
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Table 6. Composition Range of Commercial Glass Fibers, wt%
a
A
C
D
E
M
SiO2
Al2O3
B2O3
CaO
MgO
Li2O
Na2O
K2O
TiO2
CeO2
ZrO2
BeO
ZnO
Fe2O3
F2
SO3
72–72.5
0.6–1.5
60–65
2–6
2–7
13–16
3–4
74.5
0.3
22.0
0.5
52–56
12–16
8–13
16–25
0–6
53.7
64.3–65
24.8–25
12.9
9.0
3.0
0.01
22
10.0–10.3 2.6
9–10.0
2.5–3.5
13–14.2 7.5–12 1.0
0–2 0–1.3
0–1
S
EC/ Zglass
816 CEMFIL ARG
Glass
0–0.27
58
11
1.0
0–0.4 7.9–8.0
3.0
2.0
8.0
71
1
60.7
1
11
1.3
14.5
2.0
2.2
16
21.5
2.8
0.05–0.4
0–0.5
0.7
0.5
0.02
0.1
a
Ref. 146.
Strength and Fatigue. The ‘‘inert intrinsic strength’’ of silica fibers is
14 GPa (147). This term has been operationally defined as the strength of
flaw-free glass measured under conditions where no delayed failure is allowed.
This strength has been measured for few other glass compositions. For example,
iron-phosphate glasses for use as nuclear waste glass (148) show high Young’s
modulus and tensile strength. The combination of high strength and good chemical durability of the iron-phosphate glasses are valuable advantages for potential
technological applications (149).
While the measurement of MOE (modulus of elasticity) of silicate glasses is
straightforward, the calculation of strength is not similarly possible as strength
is a ‘‘weakest link’’ property. It depends not on the average properties of the sample (ie, properties of the network), but on the weakest portion of the sample. In
the case where flaws are present, the strength is governed by the critical stress
as in the Griffith equation:
¼ ðE =cÞ1=2
ð6Þ
or as in the fracture mechanics modification:
¼ KIC =Yc1=2
ð7Þ
applicable to the behavior of specimens containing sharp flaws or cracks of length
c. The parameter Y describes the geometry of the tip, E is the MOE (modulus of
elasticity/Young’s modulus), g is the fracture surface energy, and KIC is the
fracture toughness. Cracks concentrate the stress so that it may be orders of
magnitude greater at the crack tip than the applied stress. If the applied stress
is not the critical stress, then failure will not occur instantaneously. If there is
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Table 7. Properties of Commercial Fibersa
Fiber Type
Property
A
C
D
E
M
S
specific gravity
2.50
2.49
2.16
2.54–2.55 2.89 2.48–2.49
refraction index at
1.512
1.541
1.47
1.547
1.635
1.523
589.3 nm
dielectric constant
6.90
6.24–6.30 3.56–3.62 5.87–6.6
4.53–4.60
at 218C, 106 Hz
2.3
thermal
conductivity,
103 calcm/8Cs
specific heat,
0.19–0.21
0.175
0.192
0.176
cal/g8C
linear expansion
90
70–72
31
49–60
57
29–50
coefficient,
1078C 1
liquidus
1065–1120
1500
temperature,8C
fiberizing
1280
1270–1300
1565
temperature,8C
strain point,8C
1025
890
1140
1400
annealing point,8C
1090
970
657
810
softening point,8C 1285–1330 1380–1385
1420
1555
1775–1778
hardness, Vickers,
0.76
0.82
106 psi
Young’s modulus,
72.5
70
51.7
72.4–76
110
84–88
GPa
Poisson’s ratio
0.10–0.22
2414
2758–3103
2414
3500
3500
4600
virgin tensile
strength at room
temperature,
MPa
virgin tensile
5900
8300
strength at
liquid N2
temperature
fracture tough0.90
1.2
ness, MPa m1/2
stress corrosion
28–31
40
susceptibility
exponent
11.1
0.13
1.7
weight loss % of
14 mm diameter
fiber after 1 h
boil in H2O
1N H2SO4
6.2
0.10
48.2
0.1N NaOH
12–15.0
2.28
9.7
a
Ref. 146.
moisture present in the environment, subcritical slow crack growth (fatigue) will
occur, which is of major consequence in silica lightguide fibers. The effects of
crack size (including those generated from typical processing and handling)
and fatigue processes on glass strength are summarized in Figure 15.
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10000
Pristine, as-drawn
Failure strength, MPa
Ins
tan
1000
tan
eou
tre
tic
En
du
100
Pristine, annealed
ss
Sta
ngt
Formed glass
h
Fa
ran
ce
tigu
Used glass
e
it
Eff
ec
Fabrication
µ-scopic
damage
lim
t
Damaged
glass
10
Inherent
flaws
Structural
flaws
Visible
damage
1
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
Flaw size, m
Fig. 15. Effects of crack size (including those generated from typical processing and
handling) and fatigue processes on strength in silica glass (150).
Flaw Generation and Strengthening. Glass surfaces may be damaged by
either mechanical means or by chemical means, ie, a chemical interaction that
leads to mechanical degradation. In this case, a solid, liquid, or gas phase may
react with the glass surface forming a new product or developing residual stresses due to bonding materials with different thermal expansion coefficients.
The most common techniques for improving the strength of glass surfaces
are based on the fact that failure in glasses occurs in tension that in turn is the
result of stress concentrations due to surface flaws. Thus the reduction of tensile
stresses at the surface by superposition of a surface compression is usually very
effective (151,152). In thermal tempering, the rapidly cooled surface sets up
before the more slowly cooling interior. As the interior proceeds to cool, it places
the already set surface into compression. Ion-exchange strengthening is a process commonly used where large ions (Kþ) are exchange for smaller ions (Naþ)
in the glass surface at temperatures below the annealing temperature. The
increased volume required leads to a surface compression. Alkali-alumino silicate glasses provide high rates of ion exchange with relatively little stress relaxation (153). In general, the surface compressive stress for thermal tempering is
100 MPa, while for ion exchange is 1000 MPa. Ion exchange produces a
very steep stress gradient while in thermally tempered glass the compressive
layer may extend >20% of the thickness.
The use of coatings is another way of preventing the formation of flaws or
flaw growth. Polymer-based materials are usually applied to glass containers
both for mechanical protection and for decoration. Lightguides are also coated
with a polymer that must be applied in line as the fiber is drawn.
4. Manufacture
Glasses can be prepared by methods other than cooling from a liquid state,
including from the solid–crystalline state (ie, lunar glasses) and vapor phases
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Table 8. Technical Innovations of the Twentieth Centurya
basic glass processing
fiberglass
float glass process
ribbon machine for glass bulbs
owens suction machine (containers)
Danner process for making glass tubing
continuous melting of optical glass
continuous glass fibers
steam blown glass wool
rotary fiberizing
specialty glass items
glass lasers and fiber optics
other
glass ceramics
radiant glass–ceramic cooktops
glass microspheres
laminated glass
borosilicate laboratory and consumer glassware
large, flat-glass TV tubes
automotive solar control electrically heated
windshield
automotive tempered window 2.5 mm thick
photochromic and photosensitive glass
ceramic and glass foodware safety
glass lasers
low loss optical fibers
erbium-doped optical fiber amplifiers
ultraviolet-induced refractive index changes in glass
fiber optic sensors
bioactive glasses, ceramics and glass–ceramics
nuclear waste glasses
chemical tempering of glass products (ion exchange)
a
Ref. 157.
and by ultrafast quenching procedures: (1) melt spinning, in which molten metal
is ejected onto a rapidly spinning cylinder to form thin ribbons; (2) splat quenching, in which the melt is smashed onto an anvil by a compressed-airdriven hammer; (3) twin-roller quenching, in which the melt is forced between
two cylinders rotating in opposite directions at the same speed; (4) laser glazing,
in which a short, intense laser pulse is focused onto a very small volume of a sample; and (5) laser spin melting in which a rapidly rotating rod of the starting
material is introduced into a high power laser beam, eg, a CO2 laser, causing
molten droplets to spin off and form into small glass spheres (154). Table 8 summarizes the technical innovations of the twentieth century concerning glass
processing and new glass developments. The technological aspects of glass
making have been compiled, eg, by Tooley (155) and by Scholes (156).
Glass manufacture requires four major processing stages: batch preparation, melting and refining, forming, and postforming (Fig. 16). Silica is the
basis of most commercial glasses; however, it has a high melt viscosity, even at
temperatures close to 20008C, making melting and working extremely difficult.
Container and flat glass compositions are based on the Na2OCaOSiO2 system
(Table 9) with addition of other minor components to improve glass formation,
lower liquidus temperature, and improved durability. Borosilicate glasses have
low expansion coefficient and good thermal shock resistance that makes them
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Fig. 16.
Overview of glass manufacturing (158).
suitable for laboratory and kitchen ware. E-glass (Table 6) is an alkali free
SiO2Al2O3CaOB2O3 glass used for electrical insulation.
4.1. Glass-Manufacturing Processes. Batch Preparation. This step
refers to mixing and blending of raw materials to achieve a desired glass composition. The glass batch contains glass formers, fluxes, fining agents, stabilizers,
Table 9. Typical Glass Compositionsa
Glass
SiO2
Na2O
K2O
container glass
flat glass
borosilicate
lead crystal
72.7
72.8
80.1
54.0
13.8
12.7
4.5
0.2
0.5
0.8
0.3
12.2
a
Ref. 158.
CaO
11.0
8.1
0.1
MgO
PbO
0.1
3.8
31.8
Al2O3
1.6
1.4
2.6
0.1
B2O3
other
12.2
0.5
0.3
0.4
0.2
0.4
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Table 10. Common Glass Components
glass formers
fluxes
stabilizers
fining agents
colorants
others
boron oxide (B2O3) from borax or boric acid or from ores (colemanite,
rasorite, ulexite)
feldspars (Ca, Mg, Na, or K alumina silicates), source of alumina
lead oxides (PbO/litharge, Pb3O4/red lead), PbO source for lead glasses
silica sand (SiO2); 30–10 mesh size for containers, <200 mesh for fibers
cryolite (Na3AlF6)—also opacifier in opal glasses
lithium carbonate
potash (K2CO3), K2O source
soda ash (Na2CO3), Na2O source
spodumene (Li-aluminosilicate), melting accelerator
alumina (Al2O3)
aplite (K, Na, Ca, Mg-alumina silicate), alumina source
aragonite–limestone–calcite (CaCO3), CaO source
barium carbonate, BaO source for specialty glasses
dolomite, CaMg(CO3)2, CaO, and MgO source
litharge (PbO)
magnesia (MgO)
nepheline syenite (nepheline and feldspars), alumina source
strontium carbonate
zinc oxide
zirconia (ZrO2)
antimony oxide (Sb2O3); also decolorizing agent
arsenic oxide (As2O3); also decolorizing agent
barite (BaSO4); also flux and source of barium
calumite slag (CaAlsilicate by-product of the steel industry)
gypsum (CaSO4 2H2O)
salt cake (Na2SO4); also melting aid
sodium antimonite (2Na2O 2Sb2O5 H2O); also decolorizing agent
sodium nitrate (NaNO3); also oxidizing agent
cobalt oxide (Co2O3 CoO), strong blue colorant
chromite (FeO Cr2O3), used for green bottles
iron oxides–rouge (FeO, Fe2O3, Fe3O4)
manganese dioxide–pyrolusite (MnO2)
nickel oxide
potassium dichromate (K2Cr2O7), colorant in artware
pyrite (iron sulfide), colorant in amber glass
selenium, decolorizing agent, also used in colored glasses
tin oxides (SnO, SnO2), used in artware
caustic soda (NaOH solution), for batch wetting
cerium oxide (CeO), uv absorber for specialty glasses
fluorspar (CaF2)
and sometimes colorants (Table 10). The main raw material is high quality silica
sand (essentially quartz), which has to be carefully selected for several reasons.
The cost of transporting sand is four to five times the cost of the material, and
finer sands are more expensive than coarser sands. Using the incorrect size
sand can create melting and product quality problems. Other major sources for
glass formers are feldspar (a source of alumina) and borax or boric acid (manufacture of high temperature glass, Pyrex, fiberglass).
Fluxes are added to lower the temperature at which the batch melts. Soda
ash (Na2CO3) is the main source of sodium oxide in glassmaking. Stabilizers
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improve the chemical stability of the glass. Common stabilizers include limestone (calcite, 95% CaCO3), alumina, magnesia and barium carbonate. Fining
agents are used to minimize seeds, blisters, and bubbles. These agents include
sulfates, arsenic, antimony, fluorides, phosphates, and chlorides. The use of
Na2SO4 and a reducing agent is the most common fining system used for
soda–lime–silica glasses. Fining is a complex process that depends on the
glass viscosity and composition, raw materials, and the redox conditions. There
are a number of additives used to impart color or unique properties to the glass.
Common colorants include compounds of Fe, Cr, Ce, Co, and Ni. Amber glass is
produced using Fe2S (iron-pyrite). Both CoO and NiO are used to decolorize the
yellow-green tint from iron-contamination. When mixed with Fe and Co, Se creates a glass with a bronze color.
Another raw material is cullet or recycled glass, obtained from within the
plant and/or from outside recycling firms. Cullet may constitute 10–80% of the
batch. Cullet from outside recycling may be contaminated or of inconsistent quality and it is not generally used in applications where higher quality is required
(ie, float glass). Ceramic contaminants do not dissolve in the glass and remain as
inclusions in the final ware. Cullet is less costly than virgin materials and
reduces the energy required for melting.
Melting and Refining. Commercial melting refers to forming a homogeneous molten glass from the raw materials at temperatures between 1430 and
17008C (2600–31008F). As the batch is heated a series of processes and chemical
reactions occur, including melting, dissolution, volatilization, and oxidation–
reduction (redox reactions). The batch undergoes a four-step process in the melting furnace: melting, refining, homogenizing, and heat conditioning (Fig. 17).
Melting should be complete before the batch has gone through the first onehalf of the furnace. Melting rate depends on the furnace temperature, composition of the batch, grain size of the batch ingredients, amount and grain size of
cullet, and homogeneity of the batch.
During refining (or just fining), gas bubbles are eliminated from the molten
glass. The refining section of the furnace is usually separated from the main
Fig. 17.
Melting and refining processes (158).
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melting section by a bridgewall, while glass flows through a wall opening called
the throat. Gas bubbles (O2, SO2, H2O, N2, and CO2) are dissolved in the glass
depending on the type of glass and raw materials. Refining helps to remove these
bubbles. Compounds such as Na2SO4, NaCl, and CaF2 are used as refining
agents. The use of arsenic and antimony is decreasing for environmental reasons. Glass inclusions are also eliminated or reduced during refining.
Homogenizing occurs throughout the melting chamber and is finished when
the properties of the glass meet a given set of specifications. Factors affecting
homogeneity include temperature, time, batch composition, degree of mixing,
and possibility of reactions with the refractory furnace system. During thermal
conditioning, the glass is stabilized and brought to a uniform temperature. Thermal conditioning begins after the glass reaches its highest average temperature
in the furnace; after this time it will begin cooling to the working temperature
and forming.
Glass Forming. In this stage the molten glass begins its transformation
into the final shape. Molten glass can be molded, drawn, rolled, cast, blown,
pressed, or spun into fibers. For example, nearly all flat glass is produced
today by the float glass process. In this process, molten glass (10658C) flows
horizontally from the forehearth onto a pool of molten tin. As the hot glass passes
over the molten tin it conforms to the tin surface perfectly flat and develops a
uniform thickness with no distortion.
Glass containers are formed by transferring the molten glass into molds by
a method called gob feeding. During gob feeding, the weight and shape of the
molten glass gobs are controlled. The temperature of the molten glass is very
important to the formation of gobs. If the glass is too cool, the glass is too viscous
to transfer properly. Today most container manufactures use the IS (individual
section) machine for automatic gob feeding. The IS machine is capable of handling a variety of types and sizes of molds, and can produce containers at rates
>100/min. The Owens Illinois Company has developed an IS machine with
four banks of 10 ‘‘individual sections’’ that can produce over 500 bottles/min.
Continuous glass fibers were first manufactured during 1935 in Newark,
Ohio and started a revolution in reinforced composite materials that has
grown to consumption >3 106 tons/year worldwide. Raw materials for glass
fibers include silica, soda, clay, limestone, boric acid, and fluorspar, which are
melted in a furnace and refined during lateral flow to the forehearth. The molten
glass flows to Pt/Rh alloy bushings through individual bushing tips with orifices
ranging from 0.76 to 2.03 mm and is rapidly quenched and attenuated in air to
yield fine fibers ranging from 3 to 24 mm. Mechanical winders pull the fibers at
linear velocities up to 61 m/s over an applicator that coats the fibers with an
appropriate chemical sizing to aid processing and performance of the end product. A summary of forming methods and energy considerations, with a comprehensive review, has been issued by the U.S. Department of Energy (DOE) (158).
Postforming and Finishing Operations. After taking its final shape, the
glass product may be subjected to curing–drying (fiber glass products), tempering, annealing, laminating, and coating, polishing, decorating, cutting, or drilling. Annealing is the process of slow cooling to release stresses by the time
the glass product reaches room temperature. Strain and stresses are dependent
on how fast the glass is cooled through Tg.
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Annealing is done for all types of glasses except fibers. Tempering is used to
impart strength to glass sheets. It is accomplished by heating annealed glass just
below its softening temperature, and then rapidly quenching the glass with air.
The rapid cooling allows the glass surface to be in compression in relation with
the internal regions that continue to flow. The result is increased resistance to
bending failure.
After annealing, some types of flat glass are subjected to tempering, particularly automotive and some architectural glass. Glass that is going to be used
for automotive (ie, doors, windshields) may require bending before tempering.
The glass is then heated and bent to required shape, and quenched. Laminating,
typical for windshields, is the process of placing an organic plastic film between
two or more layers of glass. If the glass is broken, the pieces are held in place
by the plastic. All glass containers are annealed after forming much similar in
a way to that used for flat glass. However, nonuniform temperature distributions
may occur due to variation in glass thickness and shape complexity.
4.2. Glass Melting Tanks. Furnaces. In general, furnaces are classified as discontinuous or continuous. Discontinuous furnaces are used in small
glass-melting operations (small blown and pressed tableware and specialty
glasses) and are operated for short periods of time. In continuous furnaces,
the glass level remains constant, with new batch materials being constantly
added as molten glass is removed. Continuous furnaces are classified into four
categories (Table 11): direct-fired, recuperative, regenerative (Fig. 18), and electric melting: continuous furnaces can be fired by natural gas, electricity, or a
combination of both. In natural gas furnaces, the gas is burned in the combustion
space above the molten glass and the transfer of energy occurs through radiation
and convection. Electricity is introduced using electrodes that are placed directly
into the molten glass.
Several techniques are being used to increase the furnace production capacity optimizing capital-intensive changes. These include electrical boosting, oxyfuel firing, and preheat of the batch and cullet. These methods may also lower
operating costs and improve the environmental performance of the furnace. Electric boost typically provides 10–15% of the energy requirements in a furnace
and it is mostly used to increase productivity in an existing furnace, without
increasing air emissions or making major changes to the furnace. Preheat of
the cullet and batch is done using a separate burner or with heat available
from the furnace exhaust. Since the gas is hot when it enters the furnace, less
energy is required to reach melting temperatures. However, increased emissions
can result from increase cullet and batch preheating. New methods are being
tried, such as Praxair’s technology (160) where batch–cullet is fed at the top of
the preheater and ‘‘rains’’ through a heat exchanger. The batch–cullet particles
are deflected by internal baffles and are in direct contact with rising hot flue
gases.
Oxyfuel firing is used to increase combustion efficiency and reduce energy
requirements. In melting furnaces, natural gas reacts with air (21% O2 and
78% N2) where the nitrogen absorbs large amounts of heat, leaving the furnace
stack at high temperatures as NOx. The use of oxygen (either air-enriched or
pure O2) reduces stack gas volumes and heat losses. However, it has been
found that there is an increase in NaOH vapor concentration (three times
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Table 11. Furnace Types for Glass Melting
Discontinuous furnacesa
pot furnace
glass is melted in a refractory pot inside a furnace. open pots are
exposed to flame and gases and have capacities of 50–200 kg of
glass. Closed pots may have larger capacity and are used for
melting lead crystal glass and colored glasses.
day tanks
small units where the charging and melting cycle is repeated daily.
They are used for specialty glasses including opal, ruby, crystal and
sodalime glasses.
Continuous furnacesb
direct-fired furnaces units fired with natural gas producing 20–150 ton/day. The burners
(unit melters)
are controlled to generate convection currents, which create a
longitudinal temperature gradient along the furnace and glass
melt. They are used in cases where glass components could degrade
regenerator refractories (ie, specialty glasses,
borosilicates).
recuperative
refer to direct-fired furnaces that have been fitted with recuperators
furnaces
to recover heat from exhaust gases; recovering heat doubles the
thermal efficiency of the furnace; they are used in small operations
(ie, insulation fiber).
regenerative
most common furnace in the glass industry, with capacities of
furnaces
100–1000 ton/day. The furnace heat is collected in a regenerator
which is used to preheat combustion air (as high as 12608C) and
achieve higher energy efficiency.
End-port regenerative furnaces use side-by-side ports located in the
back wall of the furnace with the flame entering through one port
and traveling in u-shape over the glass melt from one side.
Regenerators are located next to each other against the backwall of
the furnace.
Side-port regenerative furnaces have exhaust ports and burners
placed on opposing sides of the furnace along with two regenerators, one on each side, with the flame traveling from one side to the
other (Fig. 18).
all-electric melters
These furnaces take advantage of the conductivity of molten
glass (the furnace must first be heated with fossil fuel and the
temperature raised prior to electrical melting). Molybdenum
electrodes are embedded in the bottom or sides of the furnace,
and pass electrical current through the refractory chamber,
melting the raw materials.
a
Used for small operations, <5 ton/day.
Used for larger operations over a period of years.
b
higher) compared to gas–air firing. This increase in soda vapor is detrimental to
superstructure refractories.
The trend toward using oxyfuel firing is steadily increasing (161) as an oxyfuel furnace can produce the same amount of glass as with gas–air, but at lower
fuel input. The glass industry is using today oxyfuel burners that require low
maintenance, non-water cooled burners capable of firing up to 3000 kW
(10 106 Btu/h). When regenerative furnaces (Fig. 18) are converted to oxyfuel
firing, the regenerator refractory structure is not needed, eliminating the
exhaust volume by as much as 75% (158).
Refractories for the Glass Industry. Today, continuous furnaces are
expected to last up to 10 years in operation. Glass-contact refractories have to
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Fig. 18. Cross-section of typical regenerative sideport glass-melting tank. Gas and
heated air enter at inlet A and the mix goes then to the burner blocks. The flame goes
into the furnace across the top of the glass batch (B) while exhaust gases are withdrawn
through the regenerator C. After a 15–20-min cycle, the process is reversed (from C to A)
which allows efficient use of energy (159).
be carefully selected to improve furnace life minimizing side reactions that would
lead to glass defects (162,163). At the flux line (glass level), surface tension and
density driven flows at the combustion-atmosphere–refractory–molten-glass
interface increases the refractory corrosion. Bubbles act similarly and promote
‘‘upward drilling’’ into downward facing surfaces such as throat, forehearth
entry blocks, and any horizontal joints. Low porosity fusion-cast refractories
(ie, AZS/alumina–zirconia–silica, a/b-Al2O3) are used for glass contact applications; however, these refractories show high thermal conductivity. Insulation
is essential except where corrosion is rapid as at the flux line where either external air or water cooling is used.
Floor refractories temperatures are lower than for sidewalls. However, molten metallic contaminants (from cullet materials) can drill into the refractories,
penetrating even joints and attacking down to insulation layers. Superstructure
refractories must resist batch dust, gas corrodants, fuel ash, and thermal shock
and erosion by flames. Silica is commonly used, as it has low cost, high hot
strength, and high corrosion resistance. However, for oxyfuel firing, fusion cast
b-Al2O3 is used as it is highly resistant to soda vapor (164). Bricks in the regenerators are subjected to a similar attack as the superstructure, and with a cycling
of temperature and corroding gases. Magnesium oxide refractories are normally
used at the top of the regenerator although fused cast AZS refractories are
also used. Aluminosilicate refractories are used where temperatures are lower.
Figure 18 summarizes the location and use of main refractories in a melter
Table 12. Selected Refractories Used in the Glass Industry
Composition, wt%
Furnace
section
superstructure
glass contact
601
regenerators
other
Refractory
Al2O3
SiO2
MgO
Cr2O3
Other
silica
<0.5
96
2.5–3 CaO
fused cast a,balumina
mullite sillimanite
95
0.5–1
4 R2Oc
60–80
18–37
1–6 R2O and
Fe2O3
standard fused cast
AZSd
32–36 48–53
11–17
high ZrO2O, AZS
39–41 45–48
10–13
ZrO2AZS
fused cast a,b-alumina
dense chrome
92–96 0.5–2.5 3–5
95
0.5–1
0–3.5
<1
MOR
MPa
Applications
crown b
20
1850
1590–1650
3.4–7
2
3200–3400
1870
24.5
crown
14–24
2200–2600
9–14
1–2 R2O
0–1
3400–3550
1750
1 R2O
0–1
3600–3700
1750
0–1
2
<18
5100–5500
3200–3400
4000
1750
1870
>1650
24.5
44
>1760
10–28
crown,
backwalls,
brestwalls
glass contact
and
regeneratore
glass contact
and superstructuree
glass contact
furnace throats
fiberglass
furnace
throatsf
checkersh
0–0.5 R2O
4 R2O
93–96 TiO2
2
0.3–0.6g 98
17–22
2800–2900
mag-chrome bricks
16–27
4–8
14–25
1600–3330 1540–1650 2.8–17 regenerators
and crown
1900
1650
3.4–5.5 burner blocks,
special
shapes
1840–1970
crown repairi
600–900 1090–1430 0.7–1.2 external
insulation
fused silica
98–99
34–36
99
56–58
1 CaO, 0.2
Fe2O3
27–53 12–28 8–14 Fe2O3
1 CaO
6–10 R2O
and Fe2O3
11–15
15–18
63–77
Pyrometric cone equivalent (165).
Ref. 166.
c
Alkalis.
d
Alumina zirconia silica refractories.
e
Ref. 167.
f
Ref. 168.
g
Amount of silica affects whether the refractory is direct bonded or chemical bonded (SiO2CaO ratio).
h
Ref. 169.
i
Ref. 170.
b
PCBa 8C
MgO
fused silica castables
insulation
firebrick (IFB)
a
ZrO2
Apparent Density kg/
porosity, %
m3
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while Table 12 summarizes some useful properties of refractories for the glass
industry.
A refractories database is being developed as part of a multiuniversity–
National Science Foundation program (within the NSF, Technology, Engineering, and Mathematics Education Digital Library Program) creating a digital
library of ceramic microstructures (DLCM) (171). This library will provide
researchers and engineers with digital images to illustrate microstructures of
a wide variety of functional ceramics. The DLCM will serve also as input to
the object oriented finite element code, OOF, developed at the National Institute
of Standards and Technology (NIST), which has been designed to calculate macroscopic properties from digital microstructure images (real or simulated) (172).
The University of Missouri-Rolla will begin to supply information in the Refractories Materials category, starting with refractories for the glass industry (167).
Sensors and Controls for Glassmakers. On-line sensors provide a direct
measure of some molten glass property: glass flow, melting rate, viscosity,
strength, color, refractory corrosion, emissions, etc, which need to be controlled
to optimize the glass-melting process. For best applications, a sensor should not
change the environment or affect the property being measured; and the sensor
should not be degraded by the environment. The advent of nontraditional methods of melting glasses will also require nontraditional on-line sensors under very
demanding conditions. This has prompted the U.S. DOE to invest heavily in
what is called ‘‘The Industries of the Future’’ to help ensure that R&D resources
are strategically allocated to maximize benefits (173).
The higher demand on quality of glass products (ie, flat panel displays), the
need to meet future legislation on wastes and emissions, and the need to improve
energy efficiency require new sensors for process control. By developing sensors
capable of withstanding the high temperature and severe corrosive environment,
the glass-making process can become more energy efficient and cost effective.
The status of sensor technology is presented in Table 13.
Table 13. Status of Sensor Technology in the Glass Industry
Parameter
to be controlled
Related property
being measured
color
redox analysis
Fe/Cr analysis
Influencing
variables
a
Sensor
batch composition sensor based on
organic
voltammetry
contamination
temperature
complex sensor
combustion
glass surface
profile operation
based on laser
space
temperature gas
parameters
doppler velociparameters
temperature gas
velocity heat
metry, radiation flux probe,
pyrometer and
gas analysis
system
temperature glass DNa line
corrosion of
NaOH vapor
melt composition
superstructure
concentration
measurement
in combustion
chamber
References
175, 176
177
178
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603
Table 13 (Continued )
Parameter
to be controlled
Related property
being measured
Influencing
variables
location in
chamber and
turbulence
corrosion of
glass-contact
refractories
fining process
flame characteristics and
combustion
parameters
Sensor
gas extraction
and chemical
analysis of
aqueous solutions by AA or
ICP
gas extraction
and in situ
analysis of
NaOH using
Na-sensitive
electrodes
(similar for
KOH)
In situ Na b-alumina thermodynamic cell
LIFF (laserinduced fragmentation
fluorescence),
also proposed
to detect KOH
LIBS (laserinduced
breakdown
spectroscopy),
also proposed
to measure
temperature
acoustic impedance glass composition sensor based on
of refractories
temperature
ultrasonics and
level
high-temperature piezoelectrics coupled
with echoimpact instrumentation
dielectric constant glass composition sensor based
of refractories
temperature
microwave
level
(radar)
techniques
redox state, in situ fining agent addi- electrochemical
measurements
sulfate analysis
tion batch redox
using HVG
temperature
sensor or
level
RAPIDOX
sensor
radicals, CO, soot air/gas or oxygen/ optical sensor
fuel ratio fuel
based on
input rate
flame spectra
analysis
References
179
180
180
181
182
183–188
189–190
191,192
193
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Vol. 12
Table 13 (Continued )
Parameter
to be controlled
Related property
being measured
Influencing
variables
air/gas or oxygen/
digital images of
fuel ratio fuel
flames and optiinput rate
cal spectroscopic
determinations
gas bubbles
glass gobs
homogeneity
pressure
temperature
viscosity
a
Sensor
sensor uses
recognition
technique is
based on computer analysis
of the images,
proposed to
measure NOx
and it can
estimate flame
temperature
ultrasonic
techniques
defect diagnosis by batch composition
acoustic methods
fuel imput temperature profile
bumer settings
melt viscosity
temperature
sensor based on
plunger
feeder plunger
image analysis
frequency
settings
Christiansenrefractive index
temperature
Shelyubskii
variation
profile in melter
method;
forced bubbling
computer
electric boosting
simulation
combustion
air leakages
available
chamber
(a commodity
pressure
product)
O2 partial pressure glass surface
Sn/SnO2 therof Sn in glass
modynamic
degradation
float bath
cell
Pt/Pt-Rh thercombustion cham- operation
parameters
mocouples
ber temperature
(a commodity
temperature of
product)
lining
glass melting
operation
‘‘smart’’ sensor
temperature
parameters
based on coupling a thermocouple with
an optical
pyrometer
in-line (rotation/
temperature
heat input glass
vibration)
composition
viscometer for
melting history
feeders
redox of batch
ceramic waveelectromagnetic
heat input glass
guide that
radiation
composition
sends a cohermelting history
ent milliredox of batch
meter-wave
signal to the
molten glass
and the reflection back to
the detector
Adapted from Ref. 174.
References
194
195
196
197
198
199
200
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Computer Modeling of Glass Melting. Several physical and mathematical modeling techniques are being implemented to investigate glass-making
processes, optimize these processes, and evaluate new or different operating conditions, including the following:
Design new manufacturing installations to reduce costs of plant construction
and plant operation while increasing furnace life.
Investigate and solve day-to-day operation problems.
Improve process efficiency, fuel efficiency, throughput rates, production
yields, and product quality (less defects).
Develop new products and processes in less time.
Ensure environmental quality and meet current or projected regulations.
Figure 19 summarizes the mathematical models used in the glass industry
(201). This section relates to Process Models, which outputs a product characteristic or a materials parameter, such as glass melt velocity and temperature.
Environmental models relate process variables to the emissions from the furnace
and can be treated as a subset of the Process Models. Control models relate controllable process variables to the critical process and product variables. One
example is the control of burners and emissions by digital analysis of flame
images (Table 13). Property models relate glass properties to the composition
(ie, databases described in the Properties section). The most sophisticated level
include atomistic and molecular modeling relating glass structure to properties
(as exposed in the Fundamentals section).
Under Process models, ‘‘Black Box’’ models are semiempirical models used
to input more complex models. ‘‘Overall Balance’’ models refer to momentum,
heat and mass flow models used to estimate pressure drop, and heat and mass
balances in glassmaking. Continuum models refer to the use of continuum
mechanics equations to fully describe a given process in glassmaking: NavierStokes, differential thermal and species balances, and phenomenological laws
describing the relationship between flux and gradients. As the glassmaking process involves several steps (melting, dissolution, glass delivering to forming
Fig. 19. Classification of mathematical models used in the glass industry (201).
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machines), an ideal mathematical model would be one that covers the various
parts of the process that are then linked into one output. Software has been written especially for modeling 3D flow and heat-transfer phenomena in glassmelting tanks and is commercially available: The Netherlands Organization
TNO (202), Glass Service Ltd. (203), and the Instituto Superior Técnico (204).
The U.S. DOE is supporting a new effort at the Argonne National Laboratory
to develop a Combustion Space and Glass Bath Furnace Simulator which will
be provided to the industrial users at no cost and support through a user center
(205).
4.3. Advanced Melting Techniques. Most manufacturing and glass
processing starts by converting raw materials into a homogeneous melt at high
temperatures as has been summarized in the sections on Glass Manufacturing
Processes and Glass Melting Tanks. Still, other methods have been used to
obtain special materials and include obtaining, eg, glass microspheres. Aluminosilicate glass microspheres (Fig. 20) that range in diameter from 1 to 100 mm
are made by a flame spraying technique. After melting, the glass is crushed to
particles of about the desired size. The particles are then passed through a suitable flame where they melt and form spherical droplets due to surface tension
(206).
Other methods include sol–gel processing (207–209); vapor deposition for
optical waveguides and optical mirrors (210–213), including nanoglass technologies (214); reactive sputtering for many special oxide glasses (ie, Ref. 215); thermal oxidation as in making passivating films of SiO2 on silicon (211); and anodic
oxidation on a metal or a semiconductor (216).
The following are being reevaluated for advanced melters: submerged combustion melting (a 6 ton/day unit is production in Ukraine) (217), the BOC convective glass melting system (CGM), which directs oxyfuel flames vertically
down onto the batch surface at the charging end of the furnace (218,219), Microwave of silicate glasses (220), induction melting (221), and plasma melting (222),
Fig. 20. Accuspheres are being produced in closed ranges up to 100 m. Courtesy of
Prof. Delbert Day, Mo-Sci Corp.
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a technique that has been used to melt iron silicates in metal recuperation units
(223) and is being used in Japan as waste incinerators 224.
Finally, the U.S. DOE is promoting a program called Next Generation
Melter to develop new melting technologies that will significantly increase the
efficiency and lower the cost of glass production (225). The first stage was conducted in 2001 and 2002. A review has been made on different methods that have
been used, or proposed, to melt glass in industry. These include PPGs P-10
(226) which is a patented melting system and currently used in only one furnace,
with no widespread application in the glass industry. The system separates the
glass-formation process into four discrete devices to optimize the following
elements: (1) raw materials, as a thoroughly mixed batch, are preheated to
enhance reaction temperatures <5408C; (2) the batch is heated with an oxyfuel
flame to a temperature that melts the batch ingredients and promotes the primary solid-state reactions of dissolving the sand and begins the conversion to
the glassy state; (3) the molten mixture is held at temperatures to allow
evolution of CO2 and H2O from raw materials and allow fluxing reactions to complete the dissolving of more refractory components; and (4) a vacuum is applied
to force refining mechanisms to remove remnant seeds.
5. Glass Recycling
Glass has lost market share to aluminum and plastics for almost four decades as
consumers are purchasing lightweight containers and throwing them away after
use. High costs of waste disposal and shrinking landfill suggest that recycling is
the appropriate approach for different waste materials (227).
Manufacturers benefit from recycling in several ways; it reduces consumption of raw materials, extends the life of plant equipment such as furnaces and
saves energy. Glass container manufacturing is an example of a closed-loop recycling; meaning old bottles and jars can be turned into new containers over and
over. This is a significant advantage in marketing this packaging to both the customers of companies and the consumers. All companies are seeking additional
quantities of cullet and can use more than is presently being generated. It must
Meet local plant specifications.
Be available in consistent quantities.
Be priced comparably with the raw materials for which it substitutes.
And, as is the case with other raw materials, cullet is subject to quality control standards that take into account its impact on the manufacturing process.
This has become increasingly critical as more and more recycling programs are
commingling glass and other materials during collection and processing.
6. Uses
Glass is commonly used in different applications such as architecture (228),
beverage containers, insulation (noise, thermal, eg) and some lesser known
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applications such as nuclear waste encapsulation (229). The newer applications
of glasses include components in solid-state batteries, electronic switches and
memories, electrophotography, solar cells, microspheres for optical strengthening
and medical uses, novel glass–ceramics (machinable and bioactive materials),
solder and sol–gel glasses, gradient index optics, communication fibers, sensors,
and nonlinear, active, and digital optics (230–232).
6.1. Container, Architecture, Insulation. Silicate glasses are commonly used in beverage containers, window panes, and automobile windshields.
However, coatings are used to obtain properties not inherent in glasses. The
most widely used is silver coatings in mirrors. Today, most demand on coatings
is on sodalime silica (SLS) glass surfaces and include architectural coatings (to
reflect ir wavelengths reducing solar gain, control of light), container coatings
(to prevent surface contact damage), and automotive coatings (window defrosters,
color enhancement, support for electrical–electronic connections). Recently,
coatings have been used in the optical fiber industry, that can strengthen the
glass as well as provide lubricity and abrasion resistance (233–234). Other developments are thin-film based products, such as liquid-crystal and electrochromic
glazing that provide occupant-adjustable optical properties in automotive applications. Most of these applications demand that the coatings have high abrasion
and chemical resistance and adhere strongly to the substrate. Coating research
continues to improve cost reduction, coating application, and understanding the
role of different treatments. Two general methods are known for preparing SLS
substrates to obtain high quality coatings and both depend on an alkali diffusion
barrier (235). In the first method, the glass surface is coated with pure SiO2 by a
sol–gel process. The SiO2 coated substrate is heated then at 5008C where some
densification occurs. A second method is gas-phase dealkalization procedure
based on the reactions with acidic gases, HCl, SO2, SO3, and DFE (1,1-difluoroethane). The sol–gel coating provides higher quality SnO2 coatings as compared
to dealkalized SLS substrates.
Container Coatings. This surface treatment lubricates glass containers
so they can be handled safely. The main process used is a cold-end coating of
polyethylene combined with a hot-end coating of SnO2 or TiO2. Lubricity is regulated by varying the amount of polyethylene. Hot-end coatings are applied before
annealing, using SnCl4 or TiCl4 or organic tin compounds by a CVD process to
produce coatings of either SnO2 or TiO2. Cold-end coatings are organic materials
applied after the annealing lehr. Materials for cold-end coatings include polyethylene, oleic acid, stearates, and silicones.
SurShield barrier material is a proprietary formulation, available from
Owens-Illinois (236). The material is an active and passive barrier for oxygen,
and substantially improves the protection against CO2 permeation (CO2 permeation varies by container design). The efficiencies of SurShield barrier material
can deliver key advantages for lowering total package costs; all while preserving
product freshness and extending shelf life.
Automotive Coatings. Thin-film glass coatings are used for various purposes: to reduce interior heat build-up and air conditioner load by reflecting solar
ir radiation; to provide heat to melt ice and frost from the windshield; to increase
reflection and reduce visible transmission for rear occupant privacy; to reduce
glare and enhance driver visibility; to serve as radio and telephone antennae;
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to provide an enhanced reflective region on the windshield for instrument display; to reduce emissivity to prevent frost build-up; to act as moisture sensors
to trigger defrost and wiper operation; and to provide matching colors to enhance
styling (237). Some coatings are being used to perform more than one function.
An electrically heated windshield that also reduces solar heat load and electrically conductive coatings for heatings, or for solar reduction, may also be used
as radio or mobile phone antennae.
Current coating technologies include on-line (a continuous process, integral
with the float glass-making process) vs off-line procedures, pyrolytic (vs ambient
temperature, vacuum vs ambient pressure, chemical vs physical deposition,
before vs after glass bending, and monolithic vs laminated). Pyrolytic and chemical processes were the first to be widely applied to automotive glass and remain
the most economical and widely used in terms of area of glass coated per year.
Technology for pyrolytic deposition on the float glass ribbon for large scale automotive applications was first introduced by Ford Motor Co. in 1977. The coatings
must be able to withstand subsequent high temperature (6008C) bending and
temperature operations without degradation.
Off-line processes offer greater flexibility in a batch mode; film chemistry;
and the control of important parameters such as temperature, pressure, and
glass speed. However, the process must be preceded by thorough washing and
drying. Off-line processes are typified by vacuum sputtering. Thick-film coatings
are applied off-line by silk screen prior to bending or tempering, high temperature steps that serve to fire the coatings.
Architectural Coatings. SPD (suspended particle devices) film allows the
production of a ‘‘smart’’ window that provides controllable degrees of light transmission. Used in conjunction with low E glass, which reflects heat and other commercially available materials, SPD smart windows can also block uv light and
promote energy efficiency. SPD refers to light-absorbing microscopic particles
that are suspended between two conductive-coated surfaces. The film is placed
between two panes of electrically conductive-coated glass or plastic. By turning
the electrical voltage up or down, the amount of light transmitted through the
glass or plastic window can be controlled (238). SPD uses an emulsion that is
enhanced by adjusting the composition of the matrix polymer and the liquid suspending medium such that these materials have a refractive index within the
range of 1.455–1.463. This adjustment, while maintaining immiscibility,
increases the affinity between the matrix and liquid suspending medium. This
allows small droplets of the liquid suspending medium to exist for substantially
longer periods of time without coalescence.
PPG SunClean Glass (239) is a coated glass product with photocatalytic and
hydrophilic properties that combine to make windows easier to maintain. The
transparent SunClean coating is applied to hot glass during the forming process,
where it forms a strong, durable bond with the glass surface. The photocatalytic
property of the coating is triggered by the sun’s uv rays, and works to slowly
break down and loosen organic dirt. At the same time, the coating’s hydrophilic
property causes water droplets to spread out and sheet over the coating’s surface
(Fig. 21). This sheeting action helps to rinse away loosened dirt. The self-cleaning
property of the glass is made possible by a durable, transparent coating of titanium dioxide (TiO2) applied during the manufacturing process. The application
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Fig. 21.
Vol. 12
PPGs self-cleaning window glass. Courtesy of PPG Corp.
process, patented by PPG, makes the coating an integral part of the outer glass
surface, providing homeowners with a durable, long-lasting product. Table 14
summarizes main architectural developments by PPG. Additionally, an overview
of the current state-of-the-art of transparent conducting oxides (TCOs) is given
by Ginley (240). The main markets for TCOs are in architectural applications, in
particular energy-efficient windows, and flat-panel displays (FPDs). Pyrolyzed
Table 14. Highlights in PPGs Residential Construction History
Year
Highlight
1883
1925
1938
The Pittsburgh Plate Glass Company is established in Creighton, Pa
PPG begins mass-producing sheet glass
Herculite tempered glass; several times more shatter resistant than plate
glass, is introduced
Twindow double-paned insulating glass is placed on the market
PPG becomes the first U.S. company to manufacture glass using the float
process
PPG introduces Sungate 100 low-E glass, the world’s first low emissivity glass
Sungate 300 low-E glass is introduced
Azurlite glass is developed, providing a low shading coefficient with high
visible light transmittance
Intercept insulating glass spacers are developed
Sungate 500 low-E glass is introduced
Sungate 1000 low-E glass is introduced
Intercept DSE insulating glass technology is launched
75th Intercept licensee obtained from Residential Glass manufacturers
Solarban 60 solar control low-E glass is introduced (formerly Sungate
1000 Low-E)
PPG introduces SunClean self-cleaning glass
1945
1963
1983
1989
1989
1992
1993
1995
1997
1999
2000
2001
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fluorine-doped tin oxide is widely used as coatings for preventing radiative heat
loss from windows. Indium tin oxide (ITO) is usually used in most FPD applications. The volume of FPDs produced, and hence the volume of TCO (ITO) coatings produced, continues to grow rapidly, with a current market value of over
$U.S. 15 109.
6.2. Medical Applications. Over the last decade, considerable attention has been directed toward the use of bioactive fixation of implants. Bioactive
fixation has been defined as ‘‘interfacial bonding of an implant to tissue by the
formation of a biologically active HAp layer on the implant surface’’ (241). Studies of various compositions of bioactive glasses, ceramics and glass–ceramics
have established that there are different levels of bioactivity, as measured by
rates of bonding to bulk implants or, alternatively the rate of osteoblastic proliferation in the presence of bioactive particulates 242.
A limited number of bioactive glass compositions containing SiO2Na2O
CaOP2O5 with <55% SiO2 exhibit a high bioactivity index that bond to both
bone and soft connective tissues and have been identified as bioglasses (243–
244). These materials have been classified as Class A, and are osteoproductive
(enhance osteoblastic activity) as well as osteoconductive (bone growth and
bond along the material surface). Materials classified as Class B only exhibit
osteoconductivity and examples include dense synthetic HAp and AW/GC.
Studies using a bioactive glass, 45S5, have found it to be osteoproductive, in
that it induced differentiation of osteoblasts and stimulated bone formation both
in vitro and in vivo (245). Moreover, ionic products released by the dissolution of
this bioglass in vitro for 4 days caused enhanced human osteoblasts proliferation
and induced insulin-like growth factor II mRNA expression (246). Elemental
analysis of the bioglass–conditioned medium during the experiment showed an
88-fold increase in Si concentration and to a lesser extent, changes in Ca and P
concentration relative to the controls. Such a material can be considered ideal for
tissue engineering as the released by-products promote desired cellular
responses. Table 15 summarizes recent medical and dental technological developments. An overview of recent applications of optical-fiber sensors use has been
presented by Baldini (247).
New glass-based materials are being developed to repair bone by mixing
crushed glass particles with a polymer. The mixture is to be injected into the
area of a crushed vertebrae or other damaged bone that then fills the cracks, gluing the broken pieces back together. Once this mixture hardens, it turns into a
bonelike substance, bonding itself to the original bone. Another method is being
devised to use biodegradable glass spheres that will be used to irradiate arthritis
joints. For example, small radioactive glass spheres, about one-fifth to one-tenth
the diameter of a human hair, can be injected into the damaged joint. Once the
radiation is delivered, the spheres gradually react with the body fluids and eventually disappear from the body, thus creating a safe way to expose a patient to
radiation, confining the entire radioactivity to the diseased joint. Similar procedures can be used to treat other ailments. Instead of using a solid glass sphere, a
hollow sphere or shell filled with a drug and injected into the body, or spread as a
cream onto the skin and gradually released into the body’s system. This type of
treatment releases the drug in a more uniform manner and targets the infection
or diseased area (248).
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Table 15. Recent U.S. Patents in Medical and Dental Applications
Assigneea
Patent no.
Short title
3M
US Biomaterials
Ivoclar AG
Jeneric/Pentron Inc.
Schott Glas
Jeneric/Pentron Inc
Degussa-Huls
GC Corporation
U. Missouri-Rolla
Ivoclar AG
US Biomaterials
TDK Corp.
Ivoclar AG
U. of Pennsylvania
Schott Glas
Ivoclar AG
Schott Glas
U. of Maryland
NA
Ivoclar AG
NA
NA
U. of Florida
6,437,019
6,423,343
6,420,288
6,403,676
6,403,506
6,375,729
6,362,251
6,355,585
6,379,648
6,342,458
6,338,751
6,306,785
6,306,784
6,303,290
6,297,181
6,280,863
6,278,896
6,244,871
6,224,662
6,200,137
6,255,477
6,197,342
6,190,684
Ionomer cement
Bioactive glass
Translucent lithium disilicate glass
Dental composites
Glass powder
Machinable glass–ceramics
Dental material
Ionomer cement
Biodegradable glass
Dental product
Bioactive glass
Living tissue replacement
Alkali silicate glass
Porous glass-like matrices
Barium-free X-ray-opaque dental glass
Translucent apatite glass ceramic
Biocompatible glass-metal
Bioactive glass compositions
Dental glass pillars
Chemically stable translucent apatite glass ceramic
Magnetic glass for separating biological material
Biologically active glass as a drug delivery system
Injectable bioactive glass in a dextran suspension
a
NA ¼ none designated.
Liver cancer is being treated today with rare earth aluminosilicate (REAS)
glass microspheres. These glasses are free of alkali oxides so their chemical durability is extremely high. The interest in REAS glasses stemmed from the need to
deliver a radioactive material into a diseased organ instead of external beam
radiation. By irradiating the organ in situ, shorter range b radiation can be
used minimizing damage to adjacent healthy tissue. REAS glasses satisfy body
requirements such as nontoxic, chemically insoluble in body fluids during treatment, and have specific radioactivity for therapeutic doses. Treatment with
radioactive YAS (yttrium aluminosilicate) glass microspheres, TheraSphere
(Fig. 22), containing b emitting Y-90, has proven to be a safe method of delivering
radiation doses which are five to seven times larger than doses from other methods irradiating the liver.
6.3. Communication and Electronics. There are several advantages
in using light pulses through silica glass fibers for telecommunications in
comparison to copper wires that require repeaters or signal boosters at intervals
of 2 km; eg, the repeaters in commercial fiber-optic systems are 30 km apart.
Also, the glass fibers are small (typically 100 mm) and more of them fit into a
cable of a given size. The glass fibers are not susceptible to electromagnetic interference, so the signal is clearer. Finally, the information carried on optical fibers
can be modulated at very high frequencies with more simultaneous transmissions being possible. Although the standard wavelength of transmission used
in silica optical fiber networks is in the ir (1.55 mm), there are applications in
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Fig. 22. TheraSphere, microspheres smaller than a human hair (1–100 mm) made from
rare earth aluminosilicate glasses to deliver large doses of beta radiation to diseased body
organs. Courtesy of Prof. Delbert Day and Mo-Sci Corporation.
which glasses transmitting to longer wavelengths are preferable. These include
nose cones for heat-seeking missiles; noninvasive monitoring of bodily fluids, eg,
analysis of blood by transmitting ir radiation through an earlobe; and lenses for
night vision equipment. Some chalcogenide and halide glasses transmit to the
far-ir region (up to 20 mm).
Light-focusing glass fibers and rods having radially parabolic refractive
index distributions are known as graded-refractive index (GRIN) devices
(249,250). GRIN glasses are used as waveguides for coupling optical fibers and
as lenses for compact photocopiers and compact disk players. The use of
graded-refractive index lenses could also reduce the number of elements needed
in complicated optical systems such as cameras and microscopes.
Other glasses, fluorozirconate glasses, for instance, transmit into the midir, and may be suitable for applications requiring relatively short lengths of fiber.
Tables 16 and 17 summarize recent technological developments regarding optical
applications and electronic applications, respectively.
Photonic Applications. Optoelectronic applications such as optical switches and modulators require materials having NLO properties; eg, the refractive indexes are nonlinear dependent on the intensity of the applied electric
field and are noticeable only high energy sources such as lasers are used. It
has been found that glasses containing small amounts of semiconducting microcrystals exhibit large optical nonlinearities (251,252). Halides and chalcogenide
glasses present potential applications in infrared optics and optoelectronics
(253).
Many organic and inorganic solids have been considered for photonic applications because of their nonlinear optical properties. Chalcogenide glasses with
nonlinear refractive index have been theoretically identified to be some such candidate materials (254,255). Another new family of glasses with high nonlinear
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Vol. 12
Table 16. Recent U.S. Patents in Optical Applications
Assigneea
Patent no.
Short title
Heraeus Quarzglas
GmbH
Sumitomo Electric
Physical Optics Corp.
Asahi Glass Co.
Shin-Etsu Chemical
Co. Ltd.
Corning Inc.
Matsushita Electric
Industrial
Hoya Corp.
Philips Electronics
N. A.
Nikon Corp.
Inst. of Phys. and
Chem. Research
NA
Corning Inc.
Schott ML GmbH
Corning Inc.
Fitel USA Corp.
Alcatel
Shin-Etsu Chemical
Co.
Electron. Telecom.
Res. Inst.
Hoya Corp.
Corning Inc.
Tosoh Corp.
Nikon Corp.
NA
6,451,719
Silica glass for excimer laser
6,449,986
6,446,467
6,451,434
6,442,978
Porous glass for optical fiber
Monolithic glass light shaping diffuser
Glass laminate, functional transparent article
Apparatus for sintering a porous glass
6,441,549
6,439,943
Glass envelope with continuous internal channels
Plasma display panel
6,434,976
6,433,471
6,432,854
6,432,278
Glass fiber
Plasma addressed liquid-crystal display with glass
spacers
Polarizing optical system
Controlling refractive index of silica glass
6,431,935
6,429,162
6,423,656
6,418,757
6,416,235
6,412,310
6,413,682
Lost glass process used in making display
Glass for high and flat gain 1.55 m optical amplifiers
Synthetic quartz glass preform
Method of making a glass preform
Glass ferrule optical fiber connectors
Gravity feeding powder to a plasma torch
Quartz glass substrate for photomask
6,413,891
Glass material for waveguide of an optical amplifier
6,413,894
6,410,467
6,405,563
6,378,340
6,374,641
Optical glass and optical product
Antimony oxide glass with optical activity
Opaque silica glass with transparent portion
Synthetic silica glass
Optical fiber by melting particulate glass in a glass
cladding tube
Oxide glass with long afterglow and accelerated
phosphorescence
Negative thermal expansion optical waveguide
substrate
Sol–gel method of preparing powder for use in forming
glass
Tellurite glass optical amplifier
Glass funnel for television tube
Fluoro glass ceramic
Infrared absorbing glass
Sumita Optical
Glass, Inc.
Corning Inc.
6,372,155
Corning Inc.
6,360,564
NTT Corp.
Schott Glas
None
Olympus Optical Co.
6,356,387
6,353,284
6,352,949
6,342,460
6,362,118
a
NA ¼ none designated.
optical properties, so-called quantum dot solids, is formed by nanocomposites
made up with microcrystallites of cadmium sulfide and cadmium selenide in a
silicate glass matrix. Various groups in the world are engaged in the preparation
of such nanocomposites via the sol–gel method (256).
Optical Fiber Sensors. Advances in optical-fiber temperature and pressure sensors have been reviewed by Grattan (257) highlighting industrial applications of fiber-optic temperature sensors. Temperature sensing is limited by the
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615
Table 17. Recent U.S. Patents in Electronic Applications
Assignee
Pat. no.
Title
Schott Glas
Hoya Corp.
6,417,124
6,442,975
Hitachi Ltd.
Sumitomo Electric
Industries
IBM
6,440,517
6,438,997
Alkali-free aluminoborosilicate glass and uses
Thin-plate glass article for information recording
medium
Glass material
Method of elongating glass preform
Hoya Corp.
Ohara KK
Schott Glas
6,430,965
6,426,311
6,420,291
Murata Manufacturing Co.
Nippon Electric
Glass
Nippon Sheet Glass
Co.
Samsung
Electronics
Sanyo Electric Co.
6,414,247
Low loss glass–ceramic composition with modifiable
dielectric constant
Glass substrate for information recording medium
Glass-ceramics for substrates
Lead silicate glass and a process for setting a reduced
surface resistance
Glass ceramic board
6,413,906
Li2OAl2O3SiO2 crystallized glass
6,413,892
Glass substrate for magnetic recording media
6,410,631
Composition for production of silica glass using
sol–gel process
Glass board used in the production of liquid-crystal
panels
Glass-ceramic wiring board
Sintered quartz glass products
Glass-coated substrates for high frequency
applications
Glasses and Glass–ceramics with high specific
Young’s modulus
Nanochannel glass replica membranes
Glass–ceramics substrate for information recording
medium
Barium borosilicate glass and glass–ceramic
composition
Laminated glass substrate structure
Quartz glass products and methods for making
Glass substrate having transparent conductive film
Electric or electronic module comprising a glass
laminate
High-thermal expansion glass–ceramic sintered
product
Low temperature calcined glass–ceramic and
manufacturing process
Low melting point glass, insulating package, and
sealing member
Silica glass having superior durability against
excimer laser beams
Insulating glass paste and thick-film circuit
component
6,436,332
6,400,438
Hitachi, Ltd.
6,384,347
None
6,381,986
Tyco Electronic Corp. 6,379,785
Schott Glas
6,376,402
US Navy
Nippon Sheet Glass
6,376,096
6,376,084
Asahi Glass Co.
6,362,119
Fujitsu Ltd.
None
Nippon Sheet Glass
Agfa-Gevaert
6,361,867
6,355,587
6,355,353
6,355,125
Kyocera Corp.
6,348,427
NEC Corp.
6,348,424
NEC Corp.
6,344,424
Nikon Corp.
6,339,033
Murata
6,335,298
Manufacturing Co.
maximum service temperature of the fibers. Advanced temperature and pressure
sensors are based on Bragg gratings. Optical-fiber sensors based on fiber
Bragg gratings (FBGs) provide accurate, nonintrusive, and reliable remote
measurements of temperature, strain, and pressure, and they are immune to
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Vol. 12
electromagnetic interference. FBGs are extensively used in telecommunications,
and as sensors, FBGs find many industrial applications in composite structures
used in the civil engineering, aeronautics, train transportation, space, and naval
sectors. Tiny FBG sensors embedded in a composite material can provide in situ
information about polymer curing (strain, temperature, refractive index) in a
nonintrusive way. Additionally, FBGs may be used in instrumentation as composite extensometers primarily in civil engineering applications (258,259).
6.4. NIF Laser Glass. The National Ignition Facility (NIF) (260) has
both the largest laser and the largest optical instrument ever built. The NIF
laser system uses 3100 large plates (3-ft long and about one-half as wide) of
an neodymium phosphate glass manufactured (Hoya Corporation, USA and
Schott Glass Technologies, Inc.). The main objective of the NIF optics is to
steer 192 laser beams through a 700-ft long building onto a dime-size laser-fusion
target, compressing and heating BB-sized capsules of fusion fuel to thermonuclear ignition. NIF experiments will produce temperatures and densities
like those in the Sun or in an exploding nuclear weapon. The experiments
will help scientists sustain confidence in the nuclear weapon stockpile without
nuclear tests. It will also produce additional benefits in basic science and fusion
energy.
6.5. Glasses for Nuclear Waste Disposal. Vitrification is being used
to immobilize high level nuclear waste (HLW) in a stable, chemically durable
borosilicate glass (261–266). In the waste vitrification process, the glass melt
is contained in a refractory-lined furnace. The high-temperature melt dissolves
the HLW but also corrodes the refractory. Knowledge of the corrosion resistance
of refractories to melts containing HLW is of considerable importance to the vitrification technology (267).
The borosilicate glass is being used to vitrify HLW at the Savannah River
Site in Aiken, S.C., and by West Valley Nuclear Services at West Valley, N.Y.
(268,269). Borosilicate glasses have a good chemical durability, but may not be
suitable for all HLW compositions, such as, wastes containing phosphates,
halides and heavy metals (Bi, U, Pu).
Many phosphate glasses have a chemical durability that is usually inferior
to that of most silicate and borosilicate glasses, but iron phosphate glasses are an
exception (270). In addition to their generally excellent chemical durability, iron
phosphate glasses have low melting temperature, typically between 950 and
11508C (271). Investigations of iron phosphate wasteforms obtained by adding
different amounts of various simulated nuclear wastes to a base iron phosphate
glass, whose composition is 40Fe2O3 –60P2O5 (mol %) showed that these glassy
wasteforms have a corrosion rate up to 1000 times lower than that of a comparable borosilicate glass (272–274). Generally, iron phosphate glasses can contain
up to 40 wt% of certain simulated waste. Because of their unusually high chemical durability and other properties, ironphosphate glasses, zinc–iron phosphate
glasses (275), and lead–iron phosphate glasses are of interest for nuclear waste
immobilization. The composition of high level nuclear wastes (HLW) at Hanford
from tank B-110 is shown in Table 18. The B-110 waste comes from different
steps in the bismuth phosphate process which accounts for the high concentration of Bi2O3 (276).
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617
Table 18. Simplified Composition of Hanford B-110 Waste and
Raw Materials Used to Prepare Simulated B-110 Wastea
Compound
Fe2O3
P2O5
Bi2O3
SiO2
Na2O
Al2O3
CaO
a
B-110, wt%
30.6
1.7
25.8
23.4
14.4
2.7
1.5
Raw materials used
Fe2O3
NH4H2PO4
Bi2O3
SiO2
Na2CO3
Al2O3
CaCO3
Ref. 270.
6.6. Economic Future of Glass in Construction Business. Hundreds
of private companies are active in the $13.7 109 U.S. flat and other fabricated
glass industry (eg, Cardinal IG, Fenton Art Glass, Guardian Industries, Safelite
Glass, Schott, United Glass) (277). Demand for flat glass in the United States will
approach 7 109 ft2 in 2005. Rebounding automobile production will boost
demand for laminated and tempered glass, while high energy costs and standards benefit insulating glass in the repair–improvement construction segment.
World demand for flat glass will approach 4 109 m2 in 2004, valued at
U.S. $40 109 (Asahi Glass, Pilkington, Saint-Gobain, Guardian Industries,
PPG Industries, Nippon Sheet Glass, Visteon, Vitro, Apogee Enterprises,
and Donnelly). Construction markets will grow the fastest based on expanding
global fixed investment.
U.S. lighting fixtures (U.S.$16.7 109 electric lighting fixtures industry)
demand will grow 4.8% yearly through 2006, driven by continued strength in
replacement markets where efficiency concerns generate remodeling and retrofit
projects. High efficiency products will lead gains, including electronic ballasts,
high intensity discharge (HID) lighting, light emitting diodes (LEDs) and fiber
optic systems.
Demand for glass fibers (U.S.$5.4 109 glass fiber industry, 39 key companies including Owens Corning, Johns Manville, Saint Gobain, and PPG Industries) in the United States will reach 6.8 109 lb in 2005. The best opportunities
are expected for textile glass in reinforced plastics applications based on advantages over competitive materials (eg, light weight, corrosion resistance, and
favorable cost–performance profile).
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DAVID C. BOYD
PAUL S. DANIELSON
DAVID A. THOMPSON
Corning Incorporated
MARIANO VELEZ
SIGNO T. REIS
RICHARD K. BROW
University of Missouri-Rolla